1870.] Physics. 429 



surface of the soil. In reference to this, M. E. Becquerel and others 

 have very properly observed that better and far more accurate means 

 for accomplishing this purpose exist already, and are daily employed 

 with success. 



A valuable substance for crucibles and fire-bricks has recently 

 been discovered. There occurs, in the Departement des Ardennes, 

 France, a variety of hydrated silica known by the name of gaize, 

 and geologically situated below the cretaceous deposit; the thick- 

 ness of this layer is 30 metres, and it extends over a distance of 

 24*85 English miles. The sp. gr. of this substance is 1*48 in crude 

 state, and after ignition 1*44. This stone is used as a building 

 stone ; it is, at first, quite soft, so that it can be cut with a knife. 

 The material resists a very high temperature without fusion or 

 cracking, or, also, of perceptible contraction, either cubical or linear, 

 and it has consequently been recommended for the manufacture of 

 crucibles (on the lathe), for fire-bricks, and for furnaces. 



Electricity. — A cause of error in electroscopic experiments has 

 been pointed out by Sir Charles Wheatstone, F.R.S. In the course 

 of some experiments on electrical conduction and induction the 

 author was frequently delayed by what at first appeared to be very 

 puzzling results. Occasionally he found that he could not discharge 

 the electrometer with the finger (or only to a certain degree), and 

 that it was necessary, before commencing another experiment, to 

 be in communication with a gas-pipe which entered the room. How 

 he became charged could not at that time be explained ; observation 

 and experiment, however, soon led Sir Charles to the true solution. 

 He was sitting at a table not far from the fire-place, with the electro- 

 meter (one of Peltier's construction) before him, and was engaged 

 in experimenting with discs of various substances. To ensure that 

 the one in hand (which was of tortoiseshell) should be perfectly dry, 

 it was held for a minute before the fire. Returning, and placing it 

 on the plate of the electrometer, it had apparently acquired a strong 

 charge, deflecting the index of the electrometer beyond 90°, and it 

 was then observed that the same thing took place with every disc 

 thus presented to the fire, whether of metal or any other substance. 

 The first impression was that the disc had been rendered electrical 

 by heat ; but, on placing it in contact with a vessel of boiling water, 

 or heating it by a gas-lamp, no such effect was produced. The next 

 conjecture was that the phenomenon might arise from a difference 

 in the electrical state of the air in the room, and that at the top of 

 the chimney. That this conjecture, however, was not tenable was 

 soon evident, because the same deviation of the needle of the electro- 

 meter was produced by bringing the disc near any part of the wall 

 of the room. This seemed to indicate that different parts of the 

 room were in different electrical states; but this, again, was dis- 



2 g 2 



