PHILOSOPHICAL TRANSACTIONS. 



403 



VOL. LXIH.] 



bottle; because, on exposure to the air, it deliquesces. This salt, taken moist, 

 but not very wet, and beaten to the tineness of basket sea salt, in a mortar, is to 

 be strewed to the thickness of a shilling, on a piece of tin foil, 12 inches in 

 length, and 3 in breadth. Then the foil is to be instantl)' rolled up, so as to 

 include the salt, as it lay, between the coils. The ends are to be shut by 

 pinching them together, and the whole is to be pressed flat and close. 



All this being done as quickly as possible, the first phenomenon is — A part of 

 the salt deliquesces. 2d. This part impregnated with tin, changed in colour, 

 and of a thicker consistence, begins to froth from the ends of the coil. 3d. A 

 strong frothing, accompanied with moderate warmth. 4th. The emission of 

 copious nitrous fumes. 5th. Heat intolerable to the fingers. 6th. Explosion 

 and fire, which burst and fuse the tin foil in several places, if it be very thin. 



After many conjectures and experiments. Dr. H. discovered a property in the 

 cupreous salt, from which, and the known affinities of the bodies concerned, 

 these appearances, however new and singular, may be accounted for. The 

 cupreous salt, properly dried, and placed where it may receive a heat, not much 

 greater than what the hand can bear, takes fire. The circumstances which 

 favour this ignition, and contribute to produce it in the smallest degree of heat, 

 concur in the following convenient method of trying the experiment. A piece of 

 soft bibulous paper is to be dipped in the nitrous solution .of copper, and dried 

 before the fire 2 or 3 times alternately. Then it is to be approached towards the 

 heat, as near as can be borne, by the hand which holds it, without pain: there, 

 if it has been sufficiently dried, it will presently catch fire, and burn to a 

 brown calx. 



The easy ignition of the salt in a slight heat being thus ascertained, there is 

 no room to doubt that the foregoing phenomena are produced in the following 

 manner. The acid of the liquor, which moistened the salt, quits the copper, to 

 unite with the tin, leaving the water to be imbibed by the contiguous salt of 

 copper, which then dissolves, and acts briskly on the tin foil. 



It is well known that the action of the nitrous acid on tin is always accompanied 

 with considerable heat and effervescence, and that the solution of metallic salts 

 in watry liquors is hastened by heat. In this experiment, the warmth generated 

 by the first action of the cupreous solution, promotes the deliquescence of the 

 crystallized salt. The union of the acid with the tin is rapid, not only as being 

 assisted by heat, but on account of the great surface exposed; whence the strong 

 frothing, and the extraordinary heat, by which the redundant moisture is carried 

 away, and the undecomposed part of the cupreous salt, together with that lately 

 formed with the tin, perfectly dried. 



The heat generated on both surfaces of a large expanse of tin, is concentrated 

 by closely coiling it into a small compass, and being retained by the various sur- 



