594 
MESSRS. R. T. GLAZEBROOK AND S. SKINNER ON THE 
We shall now describe the materials of which these cells were prepared. 
The zinc rods in all were of pure stick zinc from Messrs. Johnson and Matthey. 
They were cleaned with a file and sand-paper, and generally washed in a little dilute 
sulphuric acid. They were allowed to stand in distilled water until required for use, 
when they were carefully dried. 
The nature of each of the materials used in setting up the cells is indicated by 
Greek letters a ... k in the earlier columns of the Table (p. 596). 
a is a saturated solution of zinc sulphate prepared by boiling a strong solution 
with zinc oxide, filtering whilst hot to remove excess of zinc oxide, and again when 
cold to remove the basic zinc sulphate ZnO.ZnS0 4 , which had then crystallised. 
Afterwards it was evaporated to such a strength that crystals of ZnS0 4 .7H 3 0 
separated at the temperature of the room. The solution was so thoroughly 
neutralised that it only turned a blue litmus paper to a port-wine colour. Although 
great care had been thus taken to remove all basic zinc sulphate, it is very probable 
that the solution still contained some traces of this compound. 
/3 is a saturated solution of zinc sulphate prepared entirely below 30° C. Zinc 
carbonate was added to remove any excess of acid. This solution, when cooled to the 
temperature of the room, deposited crystals which were, without doubt, ZnS0 4 .7H 3 0. 
It turned a blue litmus to a port-wine colour. The reason for limiting the heating to 
30° only was to ensure that the crystals separated on cooling should contain the 
right number of molecules of water of crystallisation. Solutions which deposit 
crystals above this temperature may give hydrates containing less water. The zinc 
sulphate had been prepared according to the directions of the c British Pharmacopoeia,’ 
and was marked (Brit. Pharm.). 
For the mercurous sulphate five different specimens were tried. 
y and e are two samples of paste prepared in the same way from a specimen of 
mercurous sulphate supplied by Messrs. Harrington, of Cork. This substance was 
not pure, it contained some mercuric and basic mercuric sulphates, which was clearly 
shown by its turning bright yellow on adding water. These pastes were prepared by 
mixing at the temperature of the room some of the mercurous sulphate, a little pure 
mercury, and sufficient of the saturated zinc sulphate /3 to make a cream. This was 
vigorously shaken at intervals for two days before use. In general appearance it was 
white, with minute grey globules, which were collected in a more dense layer in the 
lower part of the bottle. 
§ and £. These pastes were prepared at the temperature of the room from very 
pure mercurous sulphate and the zinc sulphate solutions a and j8. The mercurous 
sulphate was prepared at the laboratory by precipitating pure mercurous nitrate 
(which was obtained by allowing an excess of pure mercury to stand with nitric acid), 
with sodium or potassium sulphate, when mercurous sulphate falls down as a white 
crystalline powder, and the nitrate of the alkaline metal remains in solution. This 
white precipitate is collected on a filter paper and washed with cold distilled water 
