422 Mr. Hoimrd Gruhh [April 2, 



amount of polish appears very quickly, and tlien for many hours there 

 appears to be little or no further effect. Suddenly, however, the 

 remaining greyness disappear^, and the surface is polished. The 

 reason of this is very obvious. The polisher being very inelastic, 

 polishes first only the tops of the hills, and has to abrade away all 

 the material of which these hills are composed before it reaches the 

 valleys or floors of the pits. When it does reach them, the proper 

 polish quickly appears. The second quality of pitch, that of sub- 

 sidence, is also most valuable. 



Pitch can be rendered very hard by continued boiling. By pitch 

 I mean the natural bituminous deposit which comes to us from 

 Archangel, not gas-tar pitch. It can be made so hard that it is 

 impossible to make any impression on it with the finger-nail without 

 splitting it into pieces ; and yet even in this hard condition if laid on 

 an uneven surface it will in a few days, weeks, or months subside 

 and take the form of whatever it is resting upon. The cohesion of 

 its particles is not sufficient to enable it to retain its form under 

 the action of gravity ; and as this condition is that which science tells 

 us marks the difference between solids and liquids, we must, paradoxical 

 though it may appear, class even the hardest pitch among liquid instead 

 of solid substances. 



Now how do we utilise this peculiar quality. 



The polishing tool is made by overlaying a metal or wooden disc 

 formed to nearly the required curves by a set of squares of pitch, and 

 while these are still warm pressing them against the glass, the form of 

 which they immediately take. 



In the grinding process, I showed you that the regulation of the 

 abrasion was managed partly by the character of the stroke given, and 

 partly by the local touches given to the tool by the stoning process. 



In polishing we still retain the same facilities for modifying the 

 stroke, and the same rules I gave apply generally to the polishing 

 process as well as the grinding ; but we have not got any process 

 equivalent to that of the local stoning, and even if we had it would 

 be useless, for this very quality of subsidence of the pitch would in 

 a few minutes cause any part of its surface which had been 

 reduced to come into good contact again ; we must therefore look 

 for some other means for producing more or less abrasion whenever 

 we require it. This we effect by modifying the size of the squares of 

 pitch in the various zones. Practically it is done in this way by a 

 knife and mallet. Whenever the squares are reduced, the abrasion 

 will be less. 



This is a well-known method of regulation ; but the rationale is, I 

 think, not generally understood. It is generally explained that there 

 is less abrasion because there is less abrading surface. I do not think 

 this is the true, or at least the entire, explanation. In order to under- 

 stand the action, you must conceive the pitch to be constantly in a 

 state of subsidence, the amount of that subsidence depending of course 

 on the pressure placed upon it. Now if wo reduce the size of the 



