JS6 Scientific hUeUigence. [March, 



phliret of carbon were applied to the surface, instantly the globules 

 became cased with a shell of icy spiculfK of retiform texture. 

 Where they were in contact with the water, plumose branches 

 darted from the sulphurt-t, as from a centre, to the bottom of the 

 vessel, and the whole became solidified. The sulphuret of carbon 

 in the interim volatilized, and during this period, the spicules ex- 

 hibited the colours of the solar spectrum in l)eautiful array.' 



*' If a single drop be suttered to float on the surface of' a small 

 Volume of water, it will manifest an instant reduction of tcmpe- 

 rature, but 1 could not observe with those larger globules, whtch 

 from their specific gravity sunk in the fluid, any alteration, pro- 

 vided there were none floating on the surface. 



" I have been pleai^cd, by suffering first the crust of ice to form 

 around the lower surface of the floating sulphuret, and then to 

 agitate the glass so as to dislodge the glol)ule from its cell, v. hen 

 another pelicle forms around, and entwines it, which may be again 

 displaced, and so on. 



" I am investigating further the properties and effects of this 

 curious substance, and should any particular phenomena occur, T 

 shall feel happy in communicating tlie results to you." 



V. Arsenite nf Silver. 



Arsenite of silver having been found the best means of deteciiug 

 white arsenic when in solution, I think it will be worth while to 

 state the most striking properties of this salt, as they were investi- 

 gated by Dr. Marcet, and pul)Iished by liim in a short paper in the 

 third volume of the Medico-cliirurgical Transactions. 



When this salt is first formed it has an orpiment yellow colour, 

 but when allowed to stand for some time in an open vessel, it 

 gradually becomes brown ; and this is the colour which it has when 

 dry. 



It is perfectly Insoluble in water, but dissolves readily in diluted 

 nitric acid. If an excess of amtnonia be added at the moment of 

 its formation, it is dissolved, but when it has been once dried, it 

 is no longer soluble in ammonia. 



When heated in a glass tube a white smoke evaporates, which 

 condenses on the sides of the tube in minute octahedral crystals of 

 white arsenic, while a dark brown mass remains, obviously a sub- 

 arsenite. Before the blow pipe upon a slip of platinum, and still 

 more easily upon charcoal, the silver is reduced, and the arsenic 

 totally dissipated.* 



* Dr. Rogcf, ill publisliing Dr. }ilarcet"s rem.irks on tliis test, in the second 

 volume of tlie Medico-Chirurg. Trans, p. 15S, not only referred to Mr. Hume's 

 letter on the subject, printed in the Phil. .Mag. for 1805, but gave a summary 

 a."coiint of the method tlieriin proposed. Had 1 been aware of (his circumstance, 

 when I noticed Mr. Hume's reclamation of priority in the discover3- of this test, 

 Bo far as nitrate of silver is concerned, I should have ob»erved that his anxiety on 

 this occasion was quite superflnous, since Dr. Marcet and Dr. Roget, far from 

 Laving overlooked Mr. Hume's paper, had, in the first instance, pointedly ascribeit 

 to that chemist all that belongs to him on the subject. 



