190 J. P. Cooke, Jr., on Lecture Experiments. 
even against a full atmospheric pressure, provided that the sur- 
face 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. ‘he first half of the sodium 
should be added in very small portions ata 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 
‘ 3N and H,C. Excepting the last, for which we 
have no new methods, we will now deseribe our methods of il- 
lustrating 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 [a Ha [| 2a | 
or by the molecular expression 
ay aa wer pre] 
