Prof. J. P. Cooke on Lecture Experiments. 49 



tained of a few invariable sizes. At the suggestion of the 

 author, the " Boston Belting Company " now manufacture an 

 excellent article, which they call " stopper- cord/' consisting of 

 conical rolls of very elastic rubber about 4 feet in length and vary- 

 ing in diameter from ^ an inch at one end to 1J inch at the 

 other*. From these rolls stoppers of any required size between 

 the limits named may be cut with an ordinary knife and bored 

 with a common brass cork-borer. It is only necessary to 

 moisten the tools with water in order to prevent the adhesion 

 of the rubber. Stoppers of this material are absolutely air- 

 tight even against a full atmospheric pressure, provided that the 

 surface of adhesion to the glass neck is at least half an inch in 

 length. 



Another material of prime importance in these experiments 

 is sodium-amalgam, which is most conveniently used when con- 

 taining so large an amount of sodium as to form a hard solid. 

 It is best prepared by gently heating a few ounces of metallic 

 mercury in a shallow iron dish and then cautiously adding one- 

 twentieth of its weight of sodium. The first half of the sodium 

 should be added in very small portions at a time, as the action 

 at the beginning is exceedingly violent, but soon moderates ; and 

 at last the melted amalgam must be stirred with an iron spatula 

 in order to incorporate thoroughly the materials. As soon as 

 cold, the solid amalgam readily separates from the dish, and 

 should then at once be broken up into small fragments and pre- 

 served in a well-stoppered bottle. 



The four great typical compounds of modern chemistry are 

 HC1, H 2 0, H 3 N, and H 4 C. Excepting the last, for which we 

 have no new methods, we will now describe our methods of illus- 

 trating the composition of each of these in turn. 



Hydrochloric acid. — The points to be illustrated in the case 

 of this gas are all indicated by the equation 



H 



+ 



CI 



HC1 



or by the molecular expressiou 



[ H H~] + [ciCl] = [hcT| HC1 



* After a good deal of experience, rolls of the sizes named above have 

 been found to be the most universally applicable ; but they can be made to 

 order of any desired dimension and at a cost not exceeding one dollar a 

 pound. See fig. 5, where one of these rolls is shown in the foreground of 

 the woodcut. 



Phil. Mag. S. 4. Vol. 35,. No. 234. Jan. 1868. E 



