244 Eoyal Society: — 



Batteries of this description may be had from Messrs. Tisley 

 and Spiller, Brompton Eoad. Their cost, in large numbers, is 

 about one shilling per cell, exclusive of the charge of chloride of 

 silver, which costs about two shillings per cell. The latter, either 

 in the form of powder or of rods cast upon flattened silver wire, 

 may be obtained from Messrs. Johnson and Matthey, Hatton 

 G-arden. When the battery is exhausted the reduced silver may be 

 readily reconverted into chloride, with scarcely any loss. 



April 22. — John Evans, Esq., Vice-President in the Chair. 



The following Papers were read : — 



11 On the Action of Heat on the Absorption-Spectra and Chemi- 

 cal Constitution of Saline Solutions." By Walter Noel Hartley, 

 F.C.S. 



The effects of heat on absorption-spectra were recorded in the 

 preliminary notice of this paper, published in the ' Proceedings of 

 the BoyalSociety' for 1874 (vol. xxii. p. 241). 



The contents of the present communication consist of : — 1st, his- 

 torical notes ; 2nd, method of working ; 3rd, the spectrum -measure- 

 ments of different solutions ; 4th, conclusions as to the effect of 

 heat on coloured liquids, and the following deductions as to the 

 constitution of salts when dissolved in water : — 



I. When a simple metallic salt is dissolved in water, it is not de- 

 composed in such a way that an oxide and an acid is produced, nor 

 does a compound of the metallic oxide with the acid result. 



II. When a metallic salt is dissolved in water to form a saturated 

 solution, it does not necessarily attain its maximum state of hydration. 



III. When a simple hydrated metallic salt is dissolved in water 

 to form a saturated solution, the ciystalline molecule remains 

 chemically intact, except in the case of certain compounds which 

 readily part Math their water of crystallization, when dehydration 

 takes place to form a molecule of greater stability ; or, in other 

 words, solution facilitates chemical change in this as in most other 

 cases. 



IV. When a simple salt assumes one or more definite states of 

 hydration at different temperatures below 100° C, the hydrated 

 compounds A and B will be successively produced in the liquid 

 state if a saturated solution of the original salt be heated to 100 c 

 C. ; or, in other words, the chemical constitution of the liquid is 

 altered so that, as higher temperatures are attained, it becomes a 

 solution of substance A or of substance B, at intermediate tem- 

 peratures mixtures of these. 



V. The action of heat on the violet hydrated compounds of 

 chromium is not simply a dissociation of water-molecules or of acid 

 from base, but a true decomposition, resulting in the production of 

 a different class of salts with different generic properties. 



Many new salts were prepared for this work, and others were ex- 

 amined with greater care than had previously been bestowed on 



