386 Analyses of Books. [Nov. 



to fulminating platinum, to boil the mixture to dryness, and 

 expose the residue to a heat just under redness to expel the 

 nitric acid. 



The colour of this acid is dark iron grey. It has the metallic 

 lustre, is hard enough to cut and polish brass ; but does not 

 touch steel. It is not acted on by water, nitric, sulphuric, or 

 phosphoric acid. It is insoluble in nitromuriatic acid, and in 

 cold muriatic acid ; but this last acid dissolves it by the assist- 

 ance of heat. It is insoluble in the fixed alkahes, and not 

 altered by ammonia. When heated with sulphur, sulphurous 

 acid o"as is emitted, and sulphuret of platinum formed. When 

 mixed with zinc filings, and heated, the oxide is decomposed 

 with a vivid ignition, and white oxide of zinc formed. Accord- 

 ing to Mr. Davy, seven grs. of this oxide, when heated, are 

 decomposed into six grs. of platinum, and 2-1 cubic inches of 

 oxygen gas ; while some moisture and nitric acid were disen- 

 gaged. According to this result, it is a compound of 



Platinum 100-000 



Oxygen ll'86i 



Mr. Davy considers this oxide as the protoxide, and thinks 

 that the black oxide contains 1^- times as much oxygen. He 

 informs us that he repeated some of Mr. Cooper's experiments 

 on this oxide, and found them inaccurate. 



VII. On the Methods of cutting Rock Crystal for Microme- 

 ters. By WilUam Hyde Wollaston, M.D. P.R.S.— This paper 

 being of considerable practicable utility, and very short, I con- 

 ceive that it will be better to print it entire than to attempt an 

 abridgment which might leave out something of importance to 

 the artist. It is as follows : 



For the mere purpose of examining the phenomena of double refraction, it is 

 extremely easy for any skilful workman to combine a wedge of rock crystal, or 

 anv otherdoubly refracting substance, with another wedge of crownglass opposed 

 to it, in such a manner that a luminous object seen through them shall appear in 

 its true place by ordinary refraction, accompanied by a second image at a small 

 distance, produced by the extraordinary refraction of the crystal. 



In consequence of the dispersion of colours which occurs in employing different 

 substances, the above combination is not suited for the purpose of the micrometer 

 invented by the Abbe Rochon ; but it is not difficult to obtain such a section of 

 rock crystal as may be substituted for the wedge of glass, so that the pencil of 

 light shall be restored to its original direction void of colour without diminishing 

 the separation of the images occasioned by the first wedge. 



But since the degree to which the double refraction of rock crystal separates 

 the two portions of a beam of light transmitted through it, is not so great as may 

 frequently be wished, it becomes desirable to increase this effect beyond what can 

 be produced by the most obvious method of employing that substance ; and it does 

 appear from M. llochon's-own account of hiscontrivance,* that he fully succeeded 

 in accomplishing this end. But although he informs us that the means emj)loyed, 

 as best suited to his views, had exactly the e£fect of doubling the amount of dcvia- 



loarnalde Physique, An. 9. 



