1818.] the Commencement of the Year 1817. Fart I. 25 



low a rate in this country as the muriate of potash can. If a 

 cheap mode of preparing alumina could be fallen upon, that 

 earth misrht be substituted with advantage in some of our manu- 

 factures for alum. 



3. Solution of Silver and Ammonia. — Mr. Faraday has made 

 some experiments upon the oxide of silver, obtained by dissolving 

 the common oxide of that metal in ammonia, and exposing the 

 solution to the open air. A brilliant pellicle forms on the surface, 

 which he conceives to be a peculiar oxide containing about 

 a. the quantity of oxygen that exists in the common oxide of 

 silver. His mode of operating was to put the dry oxide into a 

 tube and decompose it by heat. The weight of silver and the 

 bulk of oxygen gas evolved gave the constituents of the oxide. 

 Experiments made in this way are liable to some uncertainty 

 from the carbonic acid with which the oxide of silver is liable to 

 combine, and from the variable proportions of common air with 

 which the gaseous product is mixed. There is a method by 

 which this last source of error may be guarded against, which. 

 I have long been in the habit of employing in delicate experi- 

 ments. I put the substance from which the gas is to be 

 extracted, into a very small tubulated retort, the extremity of the 



beak of which is bent in this shape .~~~ ^^JJ- This beak is 



put into the small cup of a mercurial air holder, which I have 

 employed for many years. It was made originally according to 

 the model of Mr. Clayfield's mercurial gasometer; but Newman's 

 improved mercurial trough will answer rather better. After all 

 the gas which can be driven off by heat has been extricated, I 

 put out the lamp, and allow the apparatus to remain unmoved 

 for some hours, till the whole of the gas resumes the temper- 

 ature of the apartment. The same bulk of gas that originally 

 filled the retort will return into it, on account of the unimpeded 

 communication between the jar containing the air and the retort; 

 for I take care to prevent any mercury from lodging in the bend 

 of the beak. Thus we obtain the exact volume of gas that is 

 actually evolved, unmixed with any portion of volume derived 

 from the air in the retort. If the gas requires to be analyzed, I 

 previously ascertain the quantity of air which the retort contains, 

 and make allowance for this proportion of common air in my 

 analysis ; for it is obvious that the gas both in the retort and jar 

 will be of exactly the same quality. 



Mr. Faraday rather overrates the weight of oxygen gas. 

 According to him, 100 cubic inches of it weigh 34"072 gr. ; 

 whereas the true weight does not exceed 33*888 gr. 



From Mr. Faraday's experiments it follows, that the oxide of 

 silver with which we were previously acquainted, is composed 

 of 100 silver + 7 # 382 oxygen, and his new oxide of 100 silvef 

 + 4-704 oxygen. 



It' we were to take the mean of these two sets of experiments, 

 on the supposition that the oxygen in the protoxide is two thirds 



