Mr. Cooper's Remarks on Hydrocyanic Acid. 187 



/I, for ray A = 1-2705 ju, for ray H = 1-2301 



/x for ray H = 1-2801 it. for ray A = 1-2705 .-. S jix. _ 

 2)2-5506 8 ju, = -0096 /x - 1 ~ 



1-2753 



= = -0035 = p its dispersive power. 



With a view of ascertaining whether any and what change 

 the acid so prepared would undergo in the course of time, it 

 was put into a stoppered phial, which had also a cap or cover 

 of (rlass fitted over the stopper, so as the more effectually to 

 secure the acid from evaporation, and the phial was then put 

 into a tin box to exclude it from the action of light ; and so 

 carefully was this put aside, that I lost all trace of it until 

 a few weeks since, when upon re-examination, I found, that 

 to appearance, as far as I can remember the quantity the 

 phial contained, it has suffered no diminution in bulk, nor was 

 it perceptibly altered in appearance excepting a very minute 

 deposit of a light grey matter, which 1 believe to be lead de- 

 rived from the glass ; and on I'etaking its specific gravity and 

 its refractive index, there was found to be no appreciable dif- 

 ference from the results formerly obtained in either. 



The above facts go to prove that real hydrocyanic acid, 

 prepared by the above process, is capable of being preserved 

 for a length of time in close vessels if light be excluded ; the 

 same portion of acid is now, and has been for more than a 

 fortnight, exposed to diffuse daylight, and as yet has shown no 

 signs of decomposition ; indeed, having frequently for the pur- 

 pose of demonstration had occasion to prepare the anhydrous 

 acid, I have remarked, contrary to the general opinion, that 

 the acid so prepared, under ordinary circumstances of keeping, 

 that is, of exposure to common daylight, that not one portion 

 in five or six undergoes any change by depositing the peculiar 

 brown mattei', if the phials are perfectly well stopped. If the 

 stability of this acid should ultimately turn out to be different 

 from what is generally imagined, it is not improbable that in 

 consequence of its possessing so low a refractive index and 

 dispersive power, that it may be made available for some use- 

 ful optical purposes. 



Now it has been stated by Dr. Brewster, that the refractive 

 index of cyanogen liquefied by pressure is 1-316 ; hence it is 

 possible, 1 conceive, to deduce, without the risk of incurring 

 any very considerable error, what the refractive index of hy- 

 drogen would be if it were possible to obtain that body in a 

 liquid state; for as hydrogen and cyanogen combine in equal 

 volumes to form hydrocyanic acid, and their union occurs in 



