Chemistry and Physics. 215 



can be isolated. If now, these salts be dissolved in 120 parts of 

 water at 6°, the anhydrous salt dried at 110° sets free 4-27 calo- 

 ries, the same salt dried in a vacuum 4-21 calories, the lower hy- 

 drate 3-66 calories, and the ter-hydrate 3-44 calories. Hence (1) 

 the anhydrous salt is identical, however dried ; and (2) heat is set 

 free when a salt already abundantly hydrated, is dissolved in 

 water. From the above numbers also, it appears that the union 

 of the half molecule of water (liquid) with one molecule of the 

 anhydrous salt sets free 0-58 calory ; while the subsequent union of 

 this with the two and a half other molecules of liquid water, sets 

 free only 0*22 calory. If these values for liquid water be con- 

 verted into those for water in the solid state by subtracting from 

 them the heat of fusion of water, 0-715 for each half molecule, then 

 the curious fact appears that the union of the first half molecule 

 absorbs 0-135 calory, that of the subsequent two and a half 3-55 

 calories, while that of the three together is 3-49 calories; or in 

 other words the union of solid water to solid sodium butyrate to 

 form a crystallized hydrate, causes a considerable absorption of 

 heat, contrary to the general fact. Consequently it is clear that 

 the formation of hydrated sodium butyrate at a temperature at 

 which water is liquid, i. e., above zero, must be attended with the 

 evolution of heat, while the same hydrate produced with solid 

 water, below zero, would cause an absorption of heat in its produc- 

 tion. Berthelot calls attention to the change of sign in the heat- 

 relations produced by combination at different temperatures as be- 

 ing a fact of the same order as that observed in allotropic ele- 

 mental changes, such as for example, those of sulphur. The 

 thermic relations then of allotropic changes of state are thus 

 closely approximated to those of a chemical reaction properly so 

 called ; the stability of the bodies formed being intimately related 

 to the changes of sign in the heat-relations attending their trans- 

 formation.— 4nw. Chem. Phys., V, vi, 433, Dec, 1875. G. f. b. 



2. Action of Light on Silver Bromide, colored and uncolored. 

 — H. VoGEL has given a resume of the results of his recent experi- 

 ments upon the chemical action of light upon silver bromide both 

 pure and when mixed with some coloring matter. He finds : (1) 

 that pure silver bromide shows by sufficiently long exposure 



strong light, a sensitiveness even to the ultra-red rays— having ob- 



^ led plates showing not onlv the line A but a line beyond this, 



I distance equal to that between A and B. Silver chloride is 



„„ A and silver brom-iodide even beyond. 



(2) To the substances already mentioned, whicli increase the sensi- 

 tiveness of silver bromide for the special rays which they absorb, 

 may be added methyl-violet and cyanin, the latter increasing re- 

 markably the sensitiveness for the orange. (3) In place of putting 

 the coloring matter into the collodion as formerly, V^ogel now pre- 

 fers to flow the previously prepared plate with an alcoholic solu- 

 tion of the coloring matter which is then allowed to dry. (4) Ex- 

 periments are necessary to determine the strength of these alco- 

 holic solutions, since when they are too strong, the light is sen- 



