104 EFFECTS OF COHESION. SECT. XIV. 



Dr. Faraday has given a singular instance of cohesive 

 force inducing chemical combination, by the following 

 experiment, which seems to be nearly allied to the dis- 

 covery made by M. Dcebereiner, in 1823, of the spon- 

 taneous combustion of spongy platina (N. 159) exposed 

 to a stream of hydrogen gas mixed with common air. 

 A plate of platina with extremely clean surfaces, when 

 plunged into oxygen and hydrogen gas mixed in the pro- 

 portions which are found in the constitution of water, 

 causes the gases to combine and water to be formed, 

 the platina to become red-hot, and at last an explosion 

 to take place ; the only conditions necessary for this 

 curious experiment being excessive purity in the gases 

 and in the surface of the plate. A sufficiently pure 

 metallic surface can only be obtained by immersing the 

 platina in very strong hot sulphuric acid and then wash- 

 ing it in distilled water, or by making it the positive 

 pole of a pile in dilute sulphuric acid. It appears that 

 the force of cohesion as well as the force of affinity ex- 

 erted by particles of matter, extends to all the particles 

 within a very minute distance. Hence the platina while 

 drawing the particles of the two gases toward its sur- 

 face by its great cohesive attraction, brings them so near 

 to one another that they come within the sphere of their 

 mutual affinity, and a chemical combination takes place. 

 Dr. Faraday attributes the effect in part also to a dim- 

 inution in the elasticity of the gaseous particles on their 

 sides adjacent to the platina, and to their perfect mix- 

 ture or association, as well as to the positive action of 

 the metal in condensing them against its surface by its 

 attractive force. The particles when chemically united 

 run off the surface of the metal in the form of water by 

 their gravitation, or pass away as aqueous vapor and make 

 way for others. 



The particles of matter are so small that nothing is 

 known of their form, further than the dissimilarity of 

 their different sides in certain cases, which appears from 

 then* reciprocal attractions during crystalization being 

 more or less powerful, according to the sides they pre- 

 sent to one another. Crystalization is an effect of mole- 

 cular attraction regulated by certain laws, according to 

 which atoms of the same kind of matter unite in regu- 



