ccxc GENERAL SUMMARY, ETC. 



scale of parts is not possible, and the correctness of the di- 

 vision, as also of the total length, may be under control. 

 For these weights and measures a number of advantages 

 are claimed. The considerable hardness of the rock-crystal 

 (7) protects the weights from abrasion by usage, to which ob- 

 jection all weights of metal are open. The rock-crystal is 

 even more indifferent to the action of acids and alkalies than 

 platinum, while it is utterly indifferent to oxidation, to which 

 weights of metal are more or less liable. Moisture has no 

 effect upon it, since it is not hygroscopic. Rock-crystal, as 

 compared with the metals, has a very small co-efficient of 

 expansion, on which account the errors arising from varia- 

 tions of thermometer and barometer are reduced to a mini- 

 mum. The weights are not objectionable on the score of 

 expense, their cost being quite moderate. As produced by 

 Stern, the larger weights, from 50 grammes to 1 gramme 

 inclusive, are made of rock-crystal, the pieces having the 

 same form as the commonly used weights of gilded brass, 

 while the weights under 1 gramme are made as usual of 

 platinum. Fresenius, who has examined and employed them, 

 declares them to be admirably adapted for analytical work. 

 Lewin impregnates sandstones with a solution of sulphate 

 of alumina, which he follows with water-glass. The stones 

 thus impregnated may be polished and appear like marble. 

 They resist the action of fire and of the atmosphere, and are 

 well adapted both in appearance and durability to take the 

 place of marbles. By preparing them at a high temperature 

 the stones take on a species of glaze, which may be deco- 

 rated with a variety of colors to imitate colored marbles and 

 the like. 



