138 Prof. J. P. Cooke and Mr. F. A. Gooch on two 



the ratio , ... . 



Si : Al : ;Fe : Mg : B 

 2-75 : -88 : -17 : 1-41 : 1*26 



2-75 : 1-05 : 1-41 : 1*26 



or 2-75 : 2*46 : 1*26 



approximately 9 : 8 : 4 



The writer has described (loc. cit.) the remarkable hygroscopic 

 properties of the vermiculites, and the difficulty of separating 

 the constitutional from the hygroscopic water which they may 

 contain. The varieties from Lerni and Pelham offer the same 

 difficulty in the determination of their water, thirty to forty 

 hours being required to bring one or two grammes of either of 

 them to a constant weight at 100° C. 



In obtaining a constant temperature of 100° C, an electric 

 regulator was used which differs from other similar forms of ap- 

 paratus in simplicity of construction. The current is made or 

 broken by a very slight rise or fall of mercury in a U-tube con- 

 nected with a glass bulb within the air-bath. By means of a 

 pressure-tap which closes an open L of the connecting tube, the 

 air within the bulb may be confined as soon as the bath reaches 

 the required temperature. After this a very slight increase of 

 temperature raises the mercury column sufficiently to close the 

 electric circuit, and then the current shuts the cock which regu- 

 lates the supply of gas to the burner under the bath. The chief 

 advantage and the novelty of the apparatus is to be found in the 

 simplicity of this stopcock, which was suggested by Professor 

 H. B. Hill. It consists of an ordinary chloride-of-calcium tube 

 placed horizontally, and closed at the larger end by a rubber 

 stopper which allows a considerable freedom of motion to a 

 smaller glass tube passing through it ; by this the illuminating- 

 gas enters the chloride-of-calcium tube, from which it passes to 

 the burner. When the current is closed, an electromagnet act- 

 ing on an armature attached to the outer end of the small tube 

 plunges the curved inner end beneath the surface of some mer- 

 cury in the bulb of the chloride-of-calcium tube and thus shuts 

 off the main supply of gas, although a small orifice in the side of 

 the inner tube allows a sufficient flow to keep the flame under 

 the air-bath alive. The variation of the temperature of the air- 

 bath does not ordinarily exceed one or two degrees during periods 

 of fifteen to twenty hours, even under great variations of pressure 

 in the gas-mains. 



Table I. shows the percentages of water found in air-dried 

 specimens of the Lerni vermiculite ; and Table II. shows the per- 

 centages of water found in the same specimens dried at 100° C. 



