28 POPULAR LECTURES AND ADDRESSES. 



sphere supported within the sulphate of zinc. 

 Now, remembering that we are again at the centre 

 of the earth, and that gravity does not hinder us, 

 suppose the glass matter of the bottle suddenly 

 to become liquid sulphate of zinc, this mass would 

 become a compound sphere like the one shown on 

 this diagram (Fig. 3), and would have a radius of 

 about 8 centimetres. If it were sulphate of zinc 

 alone, and of this magnitude, its period of vibration 

 would be about 5! seconds. 



Fig. 9 shows a drop of sulphate of zinc floating 

 on a wine-glassful of bisulphide of carbon. 



In observing the phenomena of two liquids in 

 contact, I have found it very convenient to use 

 sulphate of zinc (which I find, by experiment, 

 has the same free-surface tension as \vater) and 

 bisulphide of carbon ; as these liquids do not mix 

 when brought together, and, for a short time at 

 least, there is no chemical interaction between 

 them. Also, sulphate of zinc may be made to 

 have a density less than, or equal to, or greater 

 than, that of the bisulphide, and the bisulphide 

 may be coloured to a more or less deep purple 



