188 Miscellaneous Intelligence. 



except in the magnitude of the particles. It would seem, therefore, 

 that within certain limits gunpowder will not be inflamed by par- 

 ticles of ignited iron, unless they have at least a certain magnitude 

 in relation to the magnitude of the grains of the powder. Your 

 question was probably suggested by the fact, well known to you, 

 that on putting the hand into the stream of sparks, the sensation 

 experienced is rather that of cold than of heat. This is a fact which 

 not a little surprizes those of our numerous visitors who have the 

 courage to present their hands to a stream of fire so dense as to 

 have the appearance of one continued flame. The paradox, I appre- 

 hend, may be explained in the following manner. 



The particles which make up the stream are much smaller in dimen- 

 sions and fewer in number than they appear to be, each particle, 

 from the extreme rapidity of its motion, appearing to extend several 

 inches, when, in fact, it is little more than a mere point. These 

 particles, being thus minute, do not impart a sufficient quantity of 

 heat to penetrate through the insensible external membrane of the 

 skin, called the cuticle or epidermis, so as to reach the adjacent mem- 

 brane which alone is the organ of sensation, before it is again 

 withdrawn by the increase of evaporation produced by the current 

 of air which the wheel puts in motion. If the hand is held steadily 

 in the stream until the evaporation is diminished by the gradual 

 desiccation of the skin, we shall perceive a mild sensation of heat. 

 These sensations, first of cold only, and afterwards of mild heat, 

 take place only when we present to the stream the inside of the hand 

 or fingers, where the cuticle is thick. If the back of the hand be 

 presented, a very pungent and pricking sensation of heat is pro- 

 duced at every point where a particle impinges, highly contrasted, 

 at the same time, with a general sensation of cold, produced by the 

 increased evaporation. In the first case, the heat is passing through 

 the thick cuticle of the inside of the hand, extends laterally, and 

 loses its intensity before it reaches the sensible membrane ; but the 

 cuticle on the back of the hand being extremely thin is immediately 

 penetrated *. 



12. NEW PYROPHORUS. 



Dr. Hare recommends as a new pyrophorus, Prussian blue heated 

 to redness, for about a minute, in a glass tube, and then sealed up 

 hermetically. As soon as the tube is fractured, and the contents 

 thrown out upon a table, they take fire t. 



13. CRYSTALLIZATION OF OXIDES OF IRON AND ZINC. 

 (By M. Haldat.) 



M. Haldat is in the habit of using a bundle of soft iron wire in his 

 demonstration of the action of steam at high temperatures on iron, 



* Silliman's Jouin., xvii. p. 114. f Ibid.,xix. 173. 



