standard of the adult animals, and at the end of a fortnight, which is the period when the eyes open, it reaches it perfectly.

"Thus (says he) the state of the eyes, though having no immediate connexion with the production of heat, may yet coincide with an internal structure influencing that function, and certainly furnishes signs which serve to indicate a remarkable change in this respect, since at the period of the opening of their eyes, all young mammalia have nearly the same temperature as adults." 70.

The author does not so cleverly point out any external differences in the young of the warm-blooded and of the cold-blooded birds. It is stated, indeed, that "the young birds, which are able to run about and preserve their temperature, are only covered with a tolerably thick down, and not with feathers;" but the same may be said of the others; and most annoyingly are we informed, in the very next sentence, that "the mammalia born with closed eyes, and birds hatched without feathers, produce so little heat, as to be, in relation to the air, in the state of cold-blooded animals"!! How are we to reconcile these two? But even in the case of the young of the warm-blooded birds and mammalia, there is this difference between them and the adult animals, that although the heat of the former is as high, or nearly so, and although they have the power of retaining it, when exposed to ordinary temperatures of the air, yet, if the temperature is much reduced, say to 36° or 40°, they are found to be cooled more rapidly and to a greater extent. This was easily proved by introducing them into glass vessels, surrounded with a mixture of salt and ice. We are, therefore, warranted to make the general conclusion, "that the power of producing heat, in warmblooded animals, is at its minimum at birth, and increases successively to adult age." On this principle, we may, perhaps, account for the very great mortality among new-born children in Paris, especially during the Winter months.

"It is the custom in France to convey infants, within a few hours of their birth, to the office of the mayor of the quarter in which the nativity took place, in order that the birth may be registered, and the child become possessed of its civil rights. A careful comparison of the register of births, with the register of deaths, furnished statistical observations on so large a scale, that there can be no room to doubt the correctness of the results. It appeared that the proportion of deaths, within a very limited period after birth, compared with the total births, was much greater in winter than in summer, and that this difference of propor tion, was much greater in the northern and colder departments, than in the southern and wariner. The details of this investigation are recorded in a paper which the Doctors have presented to the Institute. They have since continued the inquiry, and the following extract from a letter which I have received from Dr. Milne Edwards, will shew the accor dance of their results.

In order to ascertain in a more positive manner than before, whether the mortality of new born children is increased by their being carried to the maire immediately after birth, we obtained from the minister of the interior, necessary orders to have the tables of mortality of infants made in a certain number of parishes, where the inhabitants are scattered over a larger surface of ground; and in others, where they are, on the contrary, agglomerated around the maire. It appeared evident to us, that if our opinion was correct, the increase of mortality during Winters, must be much greater in the former parishes than in the latter, and such is, indeed, the result actually afforded by our tables.'" 476.

We come now to the consideration of the heat of adult warm-blooded

animals, and to the diversities and modifications which different animals exhibit in their power of producing, and consequently of retaining their heat; and first, of that singular tribe which hibernate at particular seasons. It is well known that some animals, on the approach of Winter, fall into a state of torpor, and death-like inactivity, and remain so till the return of warm Spring-weather; their temperature all the while being scarcely above that of the surrounding atmosphere; their respiration is feeble, irregular, occurs only at long intervals, and nutrition, at least from without, entirely ceases. The species best known in this climate are the bat, hedgehog, marmot, and several sorts of dormice.

What is the true cause of hibernation ? An experiment or two will be sufficient to determine this point. It is necessary, however, to premise, that Buffon was quite mistaken when he alleged, that the heat of hibernating animals during their state of active life in Summer, was only 54°; for in truth, they do not differ in this respect from other adult mammalia; Spallanzani, Hunter, and others, found that their temperature was from 93° to 98o. It was ascertained, however, by M. de Saissy, that their temperature falls gradually as the season advances beyond the month of August, at which time it is probably at its maximum,

"On the 6th of August, the temperature of the air being 73° Fahr. that of a marmot was almost 98° Fahr. in the axilla. On the 23d of September, the air was 64° 5 Fahr. and the marmot 88° Fahr. and on the 10th of November, the air was 44° 6' Fahr. and the animal only 81 Fahr. which is 16° 6 Fahr. lower than its temperature in the month of August. Á garden dormouse, examined under the same circumstances had a warmth of 88 Fahr. on the 3d of August; 87° Fahr. on the 23d of September; and only 69 8' Fahr. on the 10th of November, having lost 28° Fahr. since the first examination. The examínation of a hedgehog gave the following results. It was 99° Fahr. on the 3d of August; 96 Fahr. in September; and only 57° Fahr. having lost more than 40° Fahr." 77,

Reasoning upon these facts, Dr. Edwards was led to suspect, that hibernating animals, although they may exhibit, in their active state, as high a t temperature as other mammalia, have not the same power of resisting cold; and to determine this point, he instituted the following experiment.

"In April, 1819, the air being 61° Fahr. an adult bat, of the long-eared species, recently taken in good condition, and at the temperature 93 Fahr. was placed in an earthen vessel, which was cooled by a mixture of ice and salt, which surrounded it, till the air within was reduced to 33° 8 F. It had a cover, which allowed a free communication with the external air. After the animal had been there for an hour, its temperature was reduced to 57 F., being a loss in this short space of time of 36 F. Guinea-pigs, and adult birds, placed in the same circumstances, lost, at the utmost, no more than two or three degrees, although the influence of the cold was prolonged in this case, to compensate for the difference of size. We see from this, that bats are in the habit of producing less heat than animals which do not hibernate; and it is to this cause that we must chiefly ascribe the reduction of their temperature during the cold season." 79.

We have now only one step further to advance in our enquiries, after the true cause of hibernation. The following observation of Saissy gratifies our wishes. In the months of May and June, he enclosed a marmot with a little straw, in a copper box, the lid of which

"Was pierced with a hole about half an inch in diameter. After leaving it in an icehouse for twenty-four hours, he exposed it to an artificial cold of 10 c. or 18 F. below the freezing point. It fell into a profound torpor eleven hours after, and although its temperature had fallen from 35° c. or 95° F. to 5° c. or 41° F.; yet its health did not appear more altered than in the ordinary circumstances of hibernation; for, on being afterwards exposed to the warmth of the atmosphere, it recovered from its torpor, and resumed its wonted activity.

This interesting fact shews that hibernating animals may become torpid at any season, and from other causes than the want of nourishment; and also that the phenomena which take place in hibernation do not proceed from any change in the organization of the animal at the end of the summer, as some have supposed. We may now compare the hiber nating with the other mammalia, with reference to the external temperature during spring and summer, when both are in the enjoyment of the full activity and vigour of life.

In the preceding experiment, the degree and duration of the cold to which the marmot was subjected, were such as might lead us to question whether non-hibernating animals would not have lost as much heat under the same circumstances, enclosed in a box with so small an aperture, that respiration might have been impeded. We may also conclude, from the following observation of the same author, that cold was not the only influential cause:-The marmot which I reduced to a torpid state at two different times, only be came so, I believe, in consequence of its occurring to me, to close the aperture in the lid at a time when its respiration was much enfeebled. It was only in this manner that I succeeded, for all my previous attempts had been vain.' We have here, then, a combination of two causes-external cold, and diminished respiration; without being able to distinguish the respective effects of each." 79.

The general conclusion to which Dr. Edwards arrives is, that hibernation is mainly to be attributed to a deficiency in the power of producing heat, in consequence of which they are incapacitated from resisting the external cold in Winter. It is very generally believed, that all other animals maintain a steady and uniform temperature during all seasons; but we shall find that this idea is not strictly correct; and in truth there is no strongly drawn line of demarcation between the hibernating, and many other families of the mammalia. Perhaps no part of Dr. Edwards' researches is more interesting than the discovery (supposing that future experimenters confirm his statements) of the influence of the seasons on the generation of animal heat. We have already seen that frogs, and other batrachia are capable of withstanding cold much more energetically in Winter than in Summer, and that this difference is in all probability attributable, to their bodies being then more powerfully calorific; the same holds good of the higher, or warmblooded animals, including birds and the mammalia. Dr. Edwards, in the month of February, took five sparrows, and introduced them into a glass vessel, in which the air was cooled to the freezing point; they were kept in for three hours, and when examined by a thermometer, it was ascertained that the average loss of heat in that time was only 7° Fahr. The same experiment was performed in July, and at the end of the third hour, the average loss amounted to 10° Fahr. ; in August, to 83° F.; it appears, therefore, that these sparrows did not resist the cold so well in the Summer as in the Winter; and we are led by analogy (for no direct experiments are detailed) to the belief, that it is a general law of animal existence, that the power of producing heat varies with the season; that it is at its minimum point in July, and at its maximum in January, or December. It is proper however, to mention, that we have been somewhat puzzled to understand,

how the power of "producing heat" can be at its minimum in the middle of Summer, if the actual temperature of the animal is discovered to be higher. Compare the following two passages ;-at page 127, it is stated— "the power of producing heat is in Summer reduced to its minimum ;" and at page 256, we are informed, that the averages of the temperature of sparrows and yellow-hammers ran progressively

"From the depth of winter to the height of summer, within the limits of two or three degrees eent. The observation upon sparrows gave me the greatest difference. The average was in February 105° 4 Fahr.; in April 107° 6 Fahr.; in July 110° 78 Fahr. I afterwards noticed the contrary course in the decline of the year.

Hence judged, that man also would experience variations of temperature under the influence of the seasons and of the change of climate, if not to the same extent, at least within appreciable limits. Dr. John Davy, on his return from Ceylon, informed me, that the temperature of the inhabitants of that island was higher than ours by one or two degrees of Fahrenheit, and that he had observed a similar change in the same individuals before their departure and after their arrival." 257.

We need a commentary on these passages; Dr. Marshall Hall would account for the greater energy which animals possess, during Winter, of resisting cold, by referring it to diminished irritability.

If we attend to the habits and instincts of animals we shall observe many curious illustrations of this law, and also of the remark which we made above, that there are many links of connexion between the strictly hibernating animals, and those whose temperature is most steady and uniform; thus, for instance, mice, if exposed to a moderate cold in Winter are surprisingly benumbed; hence their practice of making nests, for the sake of warmth. It is by no means improbable, that even different individuals of the same species, say, for example, of mankind, vary considerably in their powers of generating heat, and consequently of withstanding any great reductions of external temperature. Unwilling to leave this subject till we have put our readers in possession of whatever is interesting and valuable in the work before us, we shall now enquire, whether any mutual connexion can be traced between the production of heat and any other function of the animal system, before proceeding to treat of asphyxia and the modifications of respiration, by age, season, and so forth. According to the plan which we have hithero pursued, we shall propound some experimental data, and thence deduce our inferences. M. de Saissy brought a bat, which was profoundly torpid, and whose temperature was 39° F. into a room where the temperature of the air was only 28° F.; he irritated it mechanically, and left it; it awoke in an hour; thirty minutes after, its heat had risen to 59°, and in another half hour to 80o.

Again, a hedgehog, equally benumbed, and whose temperature was 37°, was treated in a similar way; it awoke in two hours, when the heat was found to be 541°, and in an hour after 86°. Similar experiments were performed on dormice with similar results. If, instead of employing any mechanical stimulus to wake the animals, we merely expose them, while torpid, to a very considerable cold, the effects were nearly the same. A dormouse, whose heat was 39°, was placed in a window, exposed to a stream of very cold air, when the thermometer stood 7° below freezing; it awoke

in about an hour, and ran into its cage with agility; its temperature gradually rose to 97°. If the cold be continued too long, all the animals, viz. marmots, dormice, and hedgehogs, relapse into a fatal lethargy. What are the inferences to be drawn from these experiments? It is evident that the recovery of heat was not caused by the medium in which they were placed, since the temperature of the air was lower than that of their bodies, and that therefore it must have been a vital, not a mechanical process. It was observed, both by M. de Saissy, and by Dr. Edwards, that the respiratory movements were at first very feeble and scarcely perceptible, but increased progressively to the degree of rapidity and extent which they have, in the active state of existence; we may therefore presume that there is a mutal connexion and dependence of the two functions of respiration and production of animal heat. Having thus largely treated of some of the more interesting phenomena of the latter function, our next enquiry is to similarly investigate the former in warm-blooded animals. Dr. Edwards directs our attention in the first place, to the circumstance of different individuals of the same species, size, and age, requiring different quantities of air for the continuance of life in a given period; we shall state the experiments in his own words, to avoid all fallacy.

"I placed some sparrows, in every respect as much alike as possible, in vessels of the same form, and containing each a litre, or about a pint and three quarters of air, inverted over mercury. Still further to ensure equality, in the condition of the experiments, I performed them simultaneously, thereby avoiding differences in the temperature, pressure, and humidity of the air. In a great number of experiments, I ascertained that there was sometimes a considerable difference in the duration of life with the same quantity of air, and that the shortest and the longest duration might differ by one-third. The air, however, was altered nearly to the same degree by all, so that the duration of life was principally affected by the comparative rapidity in the consumption of oxygen." 93.

But not only do different individuals vary in this respect, but the same individuals, we have grounds to believe, at different ages. It is not easy, indeed, to experiment on the same animal at periods much remote from each other, but we may employ young and adult animals of the same species, and make allowances for differences of size. Dr. Edwards, acting on this view, took some young and some full-grown sparrows, and confined them in vessels which contained an equal quantity of air, and he found that the mean duration of the life of the former was more than 14 hours, while that of the latter did not exceed an hour and a half; a difference quite disproportionate to the difference in their bulks, and consequently in the capacity of their lungs; he therefore inferred, that a young sparrow vitiates proportionately less air than an adult one.

The experiments were repeated frequently on sparrows, having a less disparity in age and size; and the results proved to be always in accordance with the above law. Our readers will recollect that it has been already shewn, that young animals generate less heat than adults; and to this we may now add, that they consume less oxygen during respiration, even although their breathing be quicker, than in after age. Let us advance another step, and enquire, do different species of animals vitiate different quantities of air; or, in other words, do similar quantities of air support the