Intelligence and Miscellaneous Articles. 173 



fact, it suffices to calculate the value, at a fixed instant, of each of 

 the preceding impressions, and to add the results. Formulae are 

 thus obtained the discussion of which conducts to the same con- 

 sequences as observation of the facts. I have nevertheless marked 

 a difference : it is that the fixed light, of which we have the sensa- 

 tion when the velocity is very great, has in practice an intensity 

 one or two tenths below that given by the formula. This depends, 

 doubtless, on the circumstance that, in establishing the formula, 

 account has been taken of all the impressions, even of those which 

 have become too feeble isolatedly to produce a sensation, thus ad- 

 mitting that these impressions, though insensible by themselves, 

 may accumulate so as to become sensible by their sum. Experi- 

 ence seems to indicate that it is not quite so ; and if the results 

 given by it could be sufficiently precise, there would hence, per- 

 haps, result a means of determining in every case the time during 

 which the impression on the retina remains sensible. — Comptes 

 Benclus de VAcademie des Sciences, Dec. 6, 1875, pp. 1096-1098. 



ON SOME PKOPEKTIES OF GALLIUM. 

 BY LECOQ DE BOISBAUDKAN. 



After some trials which the scarcity of material rendered long 

 and laborious, I prepared some salts of gallium sufficiently pure to 

 give in the spectroscope, besides a magnificent spectrum of gallium, 

 only a feeble trace of the zinc lines Zn a 144*62 and Zn y 150'05. 

 Such a proportion of zinc is far below the limit of sensibility with 

 the ordinary reagents. 



In examining the properties of the pure salts of gallium, I have 

 noted certain divergences from what is presented when the gallium 

 is mixed with much zinc. In the main, this is not surprising ; but 

 it will demand fresh researches, with which I shall occupy myself 

 as soon as I have renewed my store of gallium, which is exhausted 

 by the following essays and what I send to-day. 



Here is first the continuation of the facts observed in mixtures 

 of gallium and zinc : — 



(18)*. Ferrocyanide of potassium appears to behave with gallium 

 as with zinc. To a dilute solution of chlorides of zinc and gallium 

 T 4 a of its volume of concentrated HC1 was added, then a slight 

 excess of yellow prussiate • lastly, to the liquor was added four 

 times its volume of water. The whole of the gallium and all the 

 zinc were precipitated. The ferrocyanides were washed with pretty 

 strong HC1, and then decomposed by sulphydrate of ammonia. 

 The chlorhydric solution of the sulphides gave the lines of Zn and 

 Ga brilliantly. 



(19) A plate of cadmium does not precipitate any thing from a 

 solution of chlorides of zinc and gallium, even after ebullition. 



(20) With fractional precipitation by carbonate of soda, with 

 boiling, of a ZnCl 2 containing gallium, the latter metal is concen- 

 trated in the first portions thrown down. The separation is so 



* See Phil. Mag. [TV .] vol. 1. p. 414. 



