142 Anali/ses of Books. [Feb. 



Dr. Wollastou represents hydrogen by 1*32, and oxygen by 10. 

 Pr. Thomson, hydrogen, 0-125, and oxygen, 1. It is much to 

 be regretted that these variations should exist ; they produce the 

 iU effect of appearing to render a subject difficult which is easy 

 of explanation, by merely stating, as Mr. Children has done, that 

 whatever portion we take of a compound mass, *' it must con- 

 tain a certain number of atoms of each substance ; and although 

 we know nothing of their actual number, still we obtain by ana- 

 lysis the proportion that the atoms of one kind bears to those of 

 the other ; or, supposing the compound to contain an equal 

 number of each, the proportionate weights of the atoms them- 

 aelves." 



On the subject of the analysis of mineral waters, Mr. Children 

 has added much to the original work of M. Thenard. He has 

 particularly noticed the opinions of the late Dr. Murray, to the 

 ingenuity of whose views on this subject, as well as on every 

 other, I wish (however useless) to add my assent. 1 confess, how- 

 ever, for reasons which I shall take another opportunity of stating, 

 that I do not accede to the correctness of his views when sup- 

 posing that those salts which exist in a mineral water are such 

 as result from the union of those acids and bases as form the 

 most soluble compounds. Whatever I may advance on this 

 subject, I am nevertheless of opinion that the results obtained 

 by Dr. Murray in his analysis of sea water, show that the mode 

 which he employed was generally correct; and Mr. Children 

 has judiciously inserted it. 



In the Appendix, Mr. Children has given Dr. Marcet's method 

 of detecting the presence of arsenic, observing *^ that the 

 unhappy frequency with which arsenic has been employed for 

 the most nefarious purposes, renders an infallible mode of 

 detecting its presence, when in very jninnte quuntityy a great 

 desideratum in medical jurisprudence.'' " It has been objected," 

 says Mr. Children, " to this test, that if a phosphate be present, 

 its indications are ambiguous; for the colour of phosphate of 

 silver is not much unlike that of arsenite of silver. An expe- 

 rienced eye, however, will readily distinguish between them ; the 

 latter being of a brighter yellow than the former." I must con- 

 fess that this ambiguity is to me an insuperable objection to 

 what is termed the silver test. I have seen precipitates occa- 

 sioned by the phosphoric and arsenious acids so similar in 

 colour that I could not distinguish any difference, and much less 

 any variation which would be a sufficient guide for deciding 

 on the solemn and unhappy occasions in which evidence is 

 required. It is, however, but proper to add, that Dr. Marcet 

 no longer depends upon the evidence afforded by colour of the 

 precipitate of arsenite of silver, unless it be corroborated by 

 other appearances. 



" The best method that I know of," says Mr. Children, "is to 

 pass a current of sulphuretted hydrogen gas into a suspected 



