Manufacture of Sicel. 115 



hy the sparks from the fine wheel. la both cases the sparks arc par- 

 ticles of ignited iron, and there can be no difierence in the two cases 

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

 that within certain limits, gunpowder will not be inflamed by particles 

 of ignited iron, unless they have at least a certain magnitude in rela- 

 tion to the magnitude of the grains of the powder. Your question 

 was probably suggested by the fact well known to you, that on put- 

 ting the hand into the stream of sparks, the sensation experienced is 

 ratlier that of cold than of heat. This is a fact which not a little sur- 

 prises 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 continuous flame. The paradox, I apprehend, may be explain- 

 ed in the following manner : — 



The particles w^hich make up the stream are much smaller in di- 

 snensions 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 tlian a mere point. These par- 

 ticles, 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 with- 

 drawn, and more than withdrawn, by the increase of evaporation, pro- 

 duced 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 sen- 

 sation 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 produced at every point where a particle impinges, highly con- 

 trasted at the same time with a general sensation of cold produced 

 by the increased evaporation. In the first case, the heat in passing 

 through the thick cuticle of the inside of the h;ind, extends laterally 

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

 die cuticle on the back of the hand, being extremely thin, is imme- 

 diately penetrated. 



Acid in Tomatos. 



I would suggest to you sir, or to Mr. Shepard, if he has more leis- 

 ure to attend to it,, the idea of examining the acid contained in To- 

 matos. I have observed that it acts powerfully on tin, which I be- 



