the discharge passes through them. Thus when the discharge first passes through the vapour of ethyl alcohol, C2HO, the spectrum on the positive side of the plate is the candle spectrum, that on the negative side the carbonic oxide spectrum. For some little time after the discharge commenced I could not detect any hydrogen lines on either side of the plate; after a time, however, they appeared on the negative side but not on the positive. If the discharge was kept running for some time without letting a fresh supply of alcohol into the tube, the "candle spectrum" on the positive side of the plate was replaced by the CO spectrum, which now occurred on both sides of the plate, accompanied on the negative side by the hydrogen spectrum. This is the appearance presented by all the compounds of carbon, oxygen, and hydrogen which I examined, when the spark had been passing through them for a considerable time, and it is what would occur if the vapour were decomposed by the spark into carbonic acid, water, and hydrogen.

The appearance of the candle spectrum on the positive side of the plate with the CO on the negative was observed in many other cases. Thus on sparking through a tube filled with CO I could not detect any difference between the spectra on the two sides of the plate, but when a little hydrogen was let into the tube the "candle spectrum" appeared on the positive side of the plate, the carbonic oxide spectrum on the negative. The same effect was observed in a tube filled with cyanogen mixed with a little hydrogen. When the tube was filled with the vapour of methyl alcohol, CH2OH, the candle spectrum was on the positive side of the plate, the carbonic oxide and hydrogen spectra on the negative; with this vapour, unlike that of ethyl alcohol, I could not detect any stage when the hydrogen spectrum was absent.

The first explanation which occurs to one of this phenomenon is that it is owing to the potential gradient at the negative side of the plate being steeper than that on the positive, so that we may imagine we have a fierce spark on the negative side, a mild one on the positive, and that the fierce spark gives the CO spectrum, the mild one the candle spectrum. There are, however, some phenomena which seem inconsistent with this explanation: in the first place if the current is reversed after flowing in one direction, traces of the former spectra linger for some time at the sides of the plates, and, secondly, if the difference is due to the greater decomposition at the negative side of the plate, how is it that in the case of the vapour of ethyl alcohol the hydrogen spectrum is not seen, at the commencement of the discharge, on the negative side of the plate? It only appears after the discharge has passed through for some time, when hydrogen has probably been set free by the decomposition of the vapour by the discharge. If the absence of the candle spectrum from the negative side

of the plate is due to the spark being so intense that the hydro-carbon which is supposed to be the origin of this spectrum cannot exist, then we ought to see the spectra of the substances which result from the decomposition of the hydro-carbon, i.e., we ought to see the hydrogen spectrum at the negative electrode. The view which seems most in accordance with the results of observations on the discharge through these vapours is that the "candle spectrum" is the spectrum of carbon when the atom is charged with negative electricity, or of some compound of carbon in which its atom is negatively charged, while the "carbonic oxide" spectrum is the spectrum of carbon when the atom is charged with positive electricity, or of some compound in which the carbon atom is positively charged.

Discharge through an Elementary Gas.-It has long been known that when the discharge passes through some elementary gases, the spectra at the two electrodes are different. This was first shown to be the case for nitrogen, then Dr. Schuster showed that the same thing occurred with oxygen, and recently Mr. Crookes has shown that it is also true in the case of argon. I have observed a very striking change in the relative brilliancy of the red and green hydrogen lines at the two electrodes. When the tube with the plate across it was filled with hydrogen at a low pressure, then on the positive side of the plate the red line tends to be brighter than the green, while on the negative side the green line tends to be brighter than the red; in some tubes this was so marked that on the positive side of the plate the red line was bright, and the green invisible, while on the negative side of the plate the green line was bright, and the red invisible. The spectroscope I was using weakened the red rays much more than the green, so that I cannot be sure that the red rays were really altogether obliterated on the negative side of the plate; the above experiment is, however, sufficient to show that on the positive side of the plate the red rays are more easily excited than the green, while on the negative side the green line is more easily excited than the red. On the negative side of the plate we have an excess of positively charged hydrogen atoms, while on the positive side of the plate there is an excess of negatively charged hydrogen atoms, and I am inclined to attribute the difference in the spectra partly at any rate to the difference in properties between a positively and a negatively charged hydrogen atom. The reason I do not attribute it wholly to the difference in the potential gradient on the two sides of the plate is that the effect is not reversed immediately, but only gradually on reversing the coil, the former spectra clinging for some time to the sides of the plate.

Chlorine. I have made a great many experiments to see if there is any difference between the spectra given by chlorine on the two

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sides of the plate, but with negative results. Chlorine seems a gas in which we might expect to find this effect, for as Dr. Schuster, in his Report on Spectrum Analysis, says, the behaviour of its spectrum indicates that we have several spectra superposed. I have not, however, been able to effect a separation of its spectra, the differences I observed between the spectra on the two sides of the plate were irregular, and due, I think, to impurities producing effects like those observed when the discharge passes through a compound gas. However, as has been mentioned before, there is even in the case of gases where distinct evidence of separation can be obtained, a region of pressure within which the effects are irregular, and I ascribe my failure to observe separation in the case of chlorine to my having failed to get the relation between the intensity of the discharge and the pressure so adjusted as to get outside this irregular region. The cases, however, in which distinct differences between the spectra of a simple gas occur at the two electrodes, seem to indicate that the spectrum given by an element is influenced by the sign of the electrical charge carried by its atoms.

I have made some experiments to determine whether there was any separation produced in a mixture of equal volumes of hydrogen and chlorine kept in the dark, when a considerable difference of potential though not sufficient to produce discharge was maintained between the two electrodes. The parts of the tube adjacent to the two electrodes could be shut off from each other by a tap, and the amount of chlorine in the two sides was determined by absorbing it by caustic potash. The mixture was at atmospheric pressure, and the electrodes were maintained at a potential difference of about 1,200 volts by connecting them to a large battery of small storage cells. The potential difference between the terminals was maintained for about 16 hours on three separate occasions, but on analysing the vessels surrounding the two electrodes, the amount of chlorine in the vessel adjacent to the negative electrode did not differ from that in the vessel adjacent to the positive electrode by more than 1 per cent., and this could be accounted for by errors of experiments, as test experiments in which the mixture had not been exposed to the electric field gave differences comparable with these. We should conclude from the preceding experiments that the molecules of a gas are not acted on by any appreciable translational force tending to move them from one place to another, when they are near to a body charged with electricity. To test this point further, two large terminals were placed in bulbs which were connected by a horizontal capillary tube, in which a drop of sulphuric acid was placed; a difference in the pressure of the gas would cause the sulphuric acid to move, and the arrangement acts as a very delicate pressure gauge. The bulbs and tube were filled with

chlorine at atmospheric pressure. The terminals were then connected to the electrodes of a battery giving a potential difference of 1,200 volts, but not the slightest movement of the drop of acid could be detected.

I wish to acknowledge the help I have received in making the preceding experiment from my assistant, Mr. E. Everett.

X. "On the Evolution of the Vertebral Column of Amphibia and Amniota." By HANS GADOW, M.A., Ph.D., F.R.S. Received June 20, 1895.

(Abstract.)

The key to the solution of the composition of the vertebral column is given by the metameric repetition of the four pairs of symmetrically arranged cartilaginous elements, the origin of which we have traced in Fishes, namely—

One pair of basidorsalia.

=

One pair of basiventralia (with its lateral outgrowths ribs or pleurapophyses, and hæmal, ventral outgrowths hæmal arches, chevrons, wedge-bones, hæmapophyses).

One pair of interdorsalia.

One pair of interventralia.

The first of these four pairs is always present and forms the neural arch. Of the other three pairs any one may be suppressed, sometimes even two in the same skleromere.

The vertebræ of the Amphibia and Amniota possess only archcentra, because the chordal sheath takes no share in their formation. According to the composition of these arco-genous centra or bodies we distinguish the following lines of evolution :

I. Earliest primitive condition. All the four pairs of arcualia are present:

1. Either as separate pieces, of which the interventralia are the smallest, e.g., Archegosaurus, Chelydosaurus, Larvæ of Urodela.

2. Or the basidorsalia and basiventralia fuse together and form the middle of the vertebral body, while the interbasalia form neutral zones of intervertebral cartilage. Such PSEUDOCENTRA are those in the tail of Urodela.

II. The interventralia are reduced.

1. The other three pairs remain separate, and the interdorsalia tend to form the centrum. (NOTOCENTROUS

type, e.g., trunk of Euchirosaurus, Actinodon, Trinerorhachis.)

2. The three pairs of basidorsalia, interdorsalia, and basiventralia co-ossify. (Trunk vertebræ of most recent Amphibia, caudal vertebræ (coccyx) of Anura.)

III. Interventralia and basiventralia are reduced. The vertebræ consist of dorsal elements only, so-called "epichordal " type. (Trunk of Pipa, Xenopus, Bombinator.)

IV. The interdorsalia are reduced. The interventralia are enlarged and form the centrum. (GASTROCENTROUS type.)

IVA. The three remaining components ossify separately and remain

separate; the basiventralia are still large and carry most of the neural arch. (All the vertebræ of Eryopse, Cricotus, the atlas of the Amniota.)

The

IVB. The centrum is much enlarged, forms the whole of the "body," and carries the neural arch (neurocentral suture). basiventralia are much reduced and form the "intervertebral disks" or menisci, attached to the cranial end of the centrum; when less reduced, they appear as the so-called wedge bones, "intercentra," or chevrons. (Trunk and tail of most Amniota.)

IVc. The centrum and the neural arches alone constitute the vertebra. The basiventrals are lost completely. (Trunk of many Amniota, for instance, the thoracic and lumbar vertebræ of lizards, caudals of many mammalia and birds.)

The phylogeny of these modifications may be expressed as follows:

The ribs are homologous structures throughout the vertebrata. They are lateral distal outgrowths of the original basiventralia, with which they later on form joints, and they ossify independently.

The " chevrons," the bony arms or arches which enclose the caudal canal, are more ventral or median outgrowths of the same basiventralia and are to be called hæmapophyses. They are possibly serially