316 Scientific Notices — -Miscellanedus. [April, 



temper, and sharpness of the cutting instrument ; much whether 

 the work is performed dry or kept constantly wet with water or 

 oil ; and also much on the disposition of the particles of iron to 

 chip. There is a great difference in different samples of iron in 

 that respect, but much more different in copper and its alloys, 

 some of which, although sufficiently soft, can scarcely be 

 wrought by turning, filing, drilling, or grinding. Whenever the 

 steel or cutting tool, from any cause, ceases to act on the iron, 

 and the heat is perhaps at the maximum, the iron, if revolving, 

 will act on the steel ; the greater the velocity the more freely it 

 acts, and the progress is marked by different appearances 

 corresponding with the different velocities. In the case of cut- 

 ting a saw plate with soft iron, if moving with a velocity barely 

 sufficient to act on the steel, this becomes heated beyond the 

 cutting tool to a blue colour ; if moving with greater velocity, 

 no change of colour is seen, except on the burr raised by the 

 tool; if with greater still, no change of colour is perceived, 

 although the movement is attended by the combustion of most 

 of the particles disengaged. These become ignited because, 

 being connected with, and forming a part of, the plate, they are 

 hy the motion disengaged with a velocity that does not admit 

 of the transmission of the heat to the other parts of the steel. 

 Perhaps the ignition is commenced, and carried to that degree 

 denominated black heat, before the particles are separated, and 

 is completed by the friction attending the separation. It is a 

 fact, perhaps not greatly known to those who have written on 

 the subject, that at the heat called black heat (but which is in fact 

 nearly or quite a red heat in the dark), steel is broken or sepu' 

 rated by fracture,^' v;ith much less force than lohen heated less or 

 more, the requisite temperature varying probably in proportion 

 to the carbon contained in the steel. 



The result of the copper wheel mentioned by MM, Darier and 

 Coliadon having no action on the steel, goes far to prove that 

 the effect depends at least as much on heat softening the steel, 

 to a certain degree, as on percussion, copper having but little 

 disposition to generate heat under any circumstances, a fact 

 duly appreciated by the manufacturers of gunpowder. 



The reason why " the heat should be nearly all concentrated 

 to the steel, and scarcely perceptible in the iron," I think to be 

 tliis; the percussion against the steel is continual, but against any 

 one part of the iron cutter, perhaps not more than from -r,^ to -^^ 

 parts of the time ; consequently the heat received by each would 

 be in an inverse proportion of the thickness of the steel to the 



* The disposition to be easily separated hy fracture at a particular heat exists in 

 carbonized or cast iron, in the alloys of copper and of tin, is very perceptible in flint 

 glass, and perhaps in all factitious metallic compounds ; some requiring a, moderate, and 

 Others a more intense beat. 



