Desiccation of Chloride of Silver. 397 



it i.« believed that the particulars of those investigations remain nearly 

 unknown, even to many of the cultivators of science who are most 

 given to such subjects in this country. The investigations themselves 

 are somewhat abstruse, and are unfortunately scattered through va- 

 rious memoirs ; but they contain the development of a principle 

 which divests the undulatory hypothesis of the only great and hitherto 

 insurmountable objection, viz. its deficiency in the explanation of the 

 unequal refrangibility of light. It is presumed, therefore, that an 

 abstract of these important researches, compiled from the several 

 memoirs of M. Cauchy, may trot be without its use to the English 

 student. Professor Powell, of Oxford, has, accordingly, been for 

 some time engaged upon the subject, and intends shortly printing a 

 concise and connected view of the theory in question., in which ex- 

 pressions are deduced of very little more complexity than in the or- 

 dinary theory, yet including the considerations which give rise to the 

 relation between the lengths of waves and the refrangibility. 



DESICCATION OF CHLORIDE OF SILVER. 



Chloride of silver, as Dr. Prout informs me, invariably gives out 

 muriatic acid at a certain stage of drying ; and he suggests whether 

 the loss may not be sufficient to influence the atomic weight of chlo- 

 rine. Into this point I have carefully examined. Nitrate of silver 

 was precipitated by muriatic acid in excess, was well washed with 

 warm water, and set to drain in a dark closet. Exposed to light it 

 acquired an acid reaction ; and heated in contact with litmus-paper, 

 the colour was reddened, even though not at the same time under the 

 influence of light. A portion was introduced into a clean retort, and 

 heat applied so as to dry it in that situation, day-light being ex- 

 cluded : the water which thus came over was quite neutral, and the 

 chloride was at length left quite white and dry, without a trace of 

 acid being lost. The same experiment was repeated with the same 

 result. 1 hence consider it as certain that pure chloride of silver may 

 be completely dried at 300° Fahr. without, loss of anv acid, if light 

 and organic matter be excluded. On heating this dry and white chlo- 

 ride in a test-tube, a portion of acid sufficient to redden litmus was 

 given out, just as the colour of the chloride darkened in the act of 

 fusing. This phenomenon is constant. To try the amount of the loss, 

 a quantity of pure chloride of silver was well dried at 300° Fahr. ; and 

 in one experiment forty-one grains, and in another ninety grains, 

 were fused in a platinum crucible. In neither case did the loss amount 

 to an appreciable quantity : 1 could not satisfy myself that it reached 

 0*01 of a grain. In these trials, however, the dry chloride was corked 

 up in the weighing flask while still warm ; for if allowed to cool in 

 the open air, it absorbs a little air and moisture, and then on being 

 fused a slight loss is perceptible. These experiments have been pre- 

 ferably made with chloride of silver thrown down from the nitrate 

 by muriatic acid ; the results are similar witli any pure chloride ; 

 but when precipitated by sea-salt, it is apt, unless very carefully 

 washed, to retain a little chloride of sodium, and then I believe the 

 developement of acid is more easy then when the chloride of silver 

 i^ quite pure. — Phil. Trans. 1883, Part II. pp. .531—5. 



