the brownish resinous matter. When pure, brom-orceid forms long white adhering needles. It has neither taste nor smell. The mother-liquors, from which it crystallizes, assume a pale reddish colour on standing, and contain some hydrobromic acid. The brom-orceid was dried in vacuo and subjected to analysis.

I. 0.355 grm. substance gave 0.5810 AgBr=0·2439 Br=68.70 per cent. bromine. II. 0407 grm. substance gave 0.665 AgBr=0·2792 Br=68.59 per cent. bromine. I. 0.5442 grm. substance gave 0.471 carbonic acid and 0.076 water.

II. 0451 grm. gave with chromate of lead 0·391 carbonic acid and 0.065 water.

The formula given above, C66 H24 Br3 O13, is purely empirical. I regret that I have been unable to determine its atomic weight by the analysis of any of its compounds, and to establish a simple relation between it and orcin, if any such exist. Bromorceid has a feeble acid reaction. It dissolves readily in alkaline solutions. These solutions soon grow dark-coloured, and when they are neutralized by muriatic acid, a yellowish resin precipitates.

Chlor-Orceid.

A similar compound is formed when chlorine is made to act upon orcin. In the course of several trials with dry orcin and with its aqueous and alkaline solutions, I obtained little more than traces of a crystalline chlorine compound. The crystals were always accompanied with a large quantity of a dark-coloured resin, which adhered to them so pertinaciously that I have not as yet been able to procure a sufficient quantity of the crystals to be able to subject them to analysis. Mr. SCHUNCK endeavoured to prepare a similar compound, but, as he has given no analysis of it, I suspect that in this instance his success has been pretty similar with my own.

Usnic Acid.

This acid, which occurs in considerable quantity in several of the lichens, was discovered by Mr. KNOP in 1843. It was also examined nearly at the same time by Messrs. ROCHLEDER and HELDT. KNOP found it in several species of Usnea, such as Usnea florida, U. hirta and U. plicata. Messrs. ROCHLEDER and HELDT extracted it from the lichen Rangiferinus, Usnea barbata and Ramalinea calicaris. In addition to these sources, I may mention that I have found it in Evernia Prunastri, where it occurs along with evernic acid, and in Ramalinea Fraxinea. KNOP's process for procuring usnic acid is by treating the lichens in a displacement apparatus with ether. This is

both a tedious and a costly method. Messrs. ROCHLEDER and HELDT extracted it by macerating the lichens with a mixture of ammonia and spirits of wine. This latter method succeeds very well, but is also costly, especially in England, where alcohol is so dear. I have found that the method already so often described, viz. macerating the lichens in milk of lime and precipitating with muriatic acid, answers perfectly well, and is much to be preferred to either of the preceding methods. It is not advisable to employ either Evernia Prunastri or any of the species of Ramalinea for the preparation of usnic acid, as the acid obtained from these lichens is always accompanied with a good deal of resinous brownish colouring matter, which adheres to the acid so pertinaciously that it is very difficult to purify it completely. The lichens I prefer for the preparation of usnic acid are the Cladonia Rangiferina, but especially the Usnea florida. The lime solution of these lichens is deep yellow, and the precipitate thrown down either by muriatic or acetic acid has a bright yellow colour. By crystallizing this precipitate repeatedly out of alcohol, aided by a little animal charcoal, the usnic acid is obtained in large flat crystals of a pale-yellow colour. Usnic acid is not readily combustible.

0.2955 grm. acid dried at 212° FAHR. and burned with oxide of copper and a stream of oxygen gas, gave 0·688 carbonic acid gas and 0·134 water.

The potash salt was prepared in the way indicated by Mr. KNOP, by boiling the acid with carbonate of potash. The salt crystallizes readily in large plates.

I. 0.407 grm. salt dried at 212° FAHR., gave 0.087 sulphate of potash =0.047 Ko =11.55 per cent.

II. 0.212 grm. salt gave 0.0453 KoSo=0·0244 Ko=11.50 per cent.

The calculated quantity of potash in the salt is 11.66 per cent.

Usnic acid is oxidized and converted into a dark brown uncrystallizable resin when it is boiled for a considerable time with an excess of either potash or baryta. The action of chlorine upon usnic acid produces a somewhat similar result.

V. On the Heat disengaged during Metallic Substitutions. By THOMAS ANDREWS, M.D., M.R.I.A., Vice-President of Queen's College, Belfast, &c. Communicated by MICHAEL FARADAY, Esq., F.R.S., &c. &c.

Received December 16, 1847.-Read January 20, 1848.

IN the present communication I propose to give an account of some new investigations on the heat disengaged in chemical actions, which may be considered a continuation of my former inquiries on the same subject*. The greater number of the experiments to be detailed in this paper were made some years ago, and the conclusion at which I arrived was briefly announced in the Philosophical Magazine for August 1844. More recently, I have taken an opportunity to repeat many of my former experiments and to add new ones on the same subject, all of which confirm the general results formerly obtained.

Having originally observed that although a very limited number of bases (potash, soda, barytes and strontia) develope nearly the same quantity of heat, when a chemical equivalent of each enters into combination with an acid, yet that the greater number of bases differ most widely from one another, when so treated, while on the other hand, that different acids (taken in the state of dilute solution) produce with the same base nearly the same amount of heat, I ventured to draw the general inference that the thermal effects produced are more intimately connected with the basic, or electro-positive, than with the acid, or electro-negative element. In conformity with this view, it appeared probable that in the decomposition of solutions of neutral salts by the addition of bases or metallic bodies, the nature of the acid or electro-negative element of the compound would exercise no special influence on the result. I have already endeavoured to establish by experiment the truth of this principle in the case of basic substitutions, and, in the present memoir, I propose to extend the same general law to the other case, in which one metallic element replaces, or is substituted for another.

Few chemical actions are more simple in their final results, or admit more easily of being varied without changing the general type of the reaction, than those which form the subject of the present inquiry. When a neutral solution of any salt of the black oxide of copper, as, for example, the sulphate, the chloride, or the acetate, is precipitated by metallic zinc, the final result is the substitution of an atom of zinc for an atom of copper in the solution, and the precipitation of an atom of copper. If * Transactions of the Royal Irish Academy, vol. xix. pp. 228, 293. Also Philosophical Transactions for 1844, p. 21.

the physical and chemical properties of equivalent solutions of different salts of copper be compared, they will be found to present almost a complete identity, and the same remark applies to the solutions of the salts of zinc which remain after the reactions are finished. We have, therefore, every condition favourable to the production of simple thermal results. For the present object, it is not necessary to inquire in what state the metallic element exists in an aqueous solution of its salts, or what changes actually occur between the first addition of the zinc and the final precipitation of the copper; it is enough to know that the final result is the same, whether we employ a solution of an oxy-salt, or of a haloid salt.

The general result of the whole investigation may be stated in the following terms:— When an equivalent of one and the same metal replaces another in a solution of any of its salts of the same order, the heat developed is always the same; but a change in either of the metals produces a different development of heat.

By the expression "solution of a salt of the same order" is understood, a solution in which the same precipitate is produced by the addition of an alkali, or, on one view of the composition of such salts, in which the metal exists in the same state of oxidation.

Salts of Copper with Zinc.

Two distinct series of experiments were made with the salts of the black oxide of copper and metallic zinc. In the first series, concentrated solutions were taken and introduced into a small glass vessel, in which was also placed a glass tube, open above, and containing pure zinc in a state of fine subdivision. The glass vessel, carefully closed, was introduced into a larger vessel of copper furnished with a lid. The latter was filled with water adjusted to the proper temperature and suspended in an outer vessel of tin plate, and the whole introduced into a cylinder closed with a lid and capable of being rotated*. After all parts of the apparatus had acquired the same temperature, a very sensible thermometer was introduced into the water contained in the copper vessel through a small orifice in the lid, and the position of the mercury in the tube observed. The thermometers having been removed and the orifice closed with a cork, the lid of the outer vessel was shut down, and the rotating wheel moved through half a revolution, by which means the metallic zinc was brought into contact with the copper solution. The rotation was afterwards continued for five minutes and a half, which was found to be sufficient not only to complete the precipitation of the copper, but also to diffuse the heat arising from the reaction uniformly through the apparatus. The temperature of the water was so adjusted as to render the corrections required for the heating and cooling influence of the air very inconsiderable; their amount was, however, ascertained in each experiment and the results altered accordingly.

* For a description and representation of a similar apparatus, see Transactions of the Royal Irish Academy, vol. xix.