Dr. Mac CuUoch on Mineralogical Hammers. 3 



trations of this law must be familiar to every one ; the pene- 

 tration of a musket ball through an open door is well known ; 

 and the same is no less true in the case of elastic fluids. Thus the 

 action of fulminating mercury will break, in the gun, that shot 

 which common gunpowder will project; and thus also, in splitting 

 rocks, the greatest effect is produced by the worst gunpowder. 

 An attention to this simple law would have prevented the use- 

 less attempts so often made to substitute the stronger detonating 

 compounds in the practice of artillery : but I must not enter, 

 further than is necessary, into this subject, though it will imme- 

 diately be seen how this doctrine bears on the use of mineralo- 

 gical hammers. 



In striking a fragment from a mass of rock, and equally, 

 indeed, in detaching the smaller superfluous parts from speci- 

 mens, it is necessary that a vibration should be excited in one 

 place, or lamina ; and, by the communication of a limited mo- 

 tion among the particles of this lamina, the parts at rest on 

 each side are separated from it. In the harder and tougher 

 stones, the nature of this process is distinctly to be seen, as it is 

 also in the more brittle and compact, as well as in glass. In the 

 former, it will be found that the point immediately subject to 

 the impulse is bruised, and that the area of vibration extends, 

 in a somewhat concentric manner, along some lamina which is 

 generally determined by the texture of the rock : in common 

 flint, and in glass, the conchoidal form of the fracture is easily 

 seen to respect the point of impulse. 



It is therefore necessary, in breaking a rock, or in detaching 

 a large fragment from a solid mass, not only that the impulse 

 should be considerable, and proportioned to the tenacity of the 

 substance, but that it should be directed on one point, or on 

 one line, or at least on a small surface. 'I'he smallness of the 

 surface of contact between the hammer and the stone, is, how- 

 ever, not only useful in this way, by causing the vibration of the 

 lamina which is to separate the adjacent parts, but it produces 

 the further effect of concentrating the whole weight, or rather 

 momentum, of the former, on one place, instead of suffering it to 

 be wasted l)y being directed on many pt)inls at once. In the form 

 B 2 



