Assaying Ores of the Precious Metals. 63 



with that of another button whose value is known, on the 

 principle that spheres are to each other as the cubes of their 

 diameters. Delicate and ingenious scales have been devised 

 for this purpose. But even if we allow that these minute 

 diameters are, in any case, measured with sufficient accu- 

 racy, there still remains the fact that the little buttons can 

 never be exact spheres, and it may well be doubted whether 

 they can be truly considered as similar solids at all. 



If then the blow-pipe is ever to rival the furnace in the 

 assaying of ores, the first thing to be secured is the ability 

 to operate with it upon larger quantities of material. 

 For this purpose a larger flame must be obtained and this 

 necessity requires that the lips of the assay er be relieved 

 from the fatigue of furnishing the air blast. An automatic 

 source of air is indispensable. An apparatus for this pur- 

 pose must be such as to furnish a steady current, under a 

 pressure which may be varied and controlled by the operator. 



A simple and inexpensive apparatus fulfilling these con- 

 ditions has been in use in my laboratory for nearly three 

 years. Its essential parts are an air-tight tank from which air 

 is to be driven, and a regulator by which the current is 

 made uniform and its force determined. 



The tank may be made of metal or of wood and may be 

 of any size or shape desirable. A wine-cask or beer-barrel 

 answers the purpose admirably. The . water, by which 

 pressure is to be obtained, must enter the side of the tank 

 near the top ; and to avoid the unpleasant rattle of falling 

 water the pipe which supplies it extends across the inte- 

 rior almost to the opposite side. A glass tube extending 

 along the height of the tank outside, both ends opening % 

 into the interior, continually announces the height of the 

 water and informs the operator of the amount of air at his 

 disposal. Besides these tubes, the tank is supplied with 

 another, leading from the top, by which the air-blast is 

 delivered, while at the bottom there is a large faucet by 



