90 Dr, B. Angus Smith, [March 25, 



ganate and permanganate. A permanganate of the strength described 

 has a dingy appearance and uncertain colour. I do not doubt that a 

 pure permanganate of a suitable strength may be obtained pleasant to 

 work with. There is some difficulty in obtaining pure water for pre- 

 paring the solution. If allowed to stand for some time with a man- 

 ganate it becomes purified. 



The solution of chameleon is apt to change, although slowly, even 

 when it is hermetically sealed in a glass tube. The solution described 

 had become nearly colourless when sealed up hermetically for about 

 three months. It is found readily to change when it is exposed to 

 air by frequent removal of the stopper of the bottle containing it. 

 Its strength must be tested occasionally ; and if it differs from the 

 standard, a calculation must be made for its reduction. The 

 strength of the permanganate solution is extremely small. A few 

 grains of the ordinary solutions of manganese used will make some 

 thousand grains of the solution here employed. The reason of this 

 lies in the extremely small amounts of organic matter found in even 

 the worst air. 



The vessel used is simply a bottle, with a perforated stopper, 

 through which pass two tubes. To one of these a stojicock is attached, 

 to the other a clasp or stopcock. The standard size proposed is 100 

 cubic inches ; and to this all the experiments have been reduced ; the 

 vessels actually used contain between 80 and 100 cubic inches of air. 

 The stopcock is of glass, or of hard caoutchouc, which is still better. 

 When the bottle is to be filled with the air to be tested, the stopper is 

 removed, and the pipe of an exhausting pump is inserted, reaching to 

 the bottom of the bottle. The pump is made like a cylindrical bellows 

 of about 8 inches long when stretched out, and about 4 in diameter, 

 and is compressible into the thickness of about two inches. The sides 

 are made of thin Mackintosh cloth. By the use of the pump the air 

 of the vessel is removed, and the external air of course enters. A few 

 strokes of the pump are sufficient, i.e. from 6 to 10. After ten strokes 

 I perceive no change, and am inclined to think that it is an unnecessary 

 number. The test liquid is poured into a graduated tube or burette, 

 containing somewhat more than will be required. A portion is then 

 poured into the tube which passes through the stopper, and the stop- 

 cock is opened to allow it to pass. Small quantities are used ; when 

 it has entered the bottle, the liquid is made to spread over the sides, 

 and time given it to be exposed to the action of the air ; it is found 

 that in five or six minutes a decided epoch is attained from which to 

 date the comparative action. 



In order to see the colour the liquid must be allowed to trickle 

 down the sides of the vessel, and collect itself at one point of the cir- 

 cumference at either end of the cylindrical part of the bottle. This 

 part must be raised up to the level of the eye »o that the longest axis 

 may be presented to the sight, and thereby the deepest shade of colour. 

 It requires some time to accustom oneself to the sight of such a small 

 amount of colour ; but when it is once well observed, it will be found to 

 be a method which will admit of the greatest precision. The first few 



