208 EXPERIMENTS ON ADHESION AND COHESION. 



a difficult operation, and it is therefore best to purchase the mercui 

 of the purest quality, and carefully to avoid bringing it in contact wil 

 other metals, such as zinc, lead, etc., but especially with gold am 

 silver, for in the latter case not only the mercury, but also the golc 

 and silver will be spoiled ; valuable articles of gold and silver shoi 

 hence be put away while mercury is being handled. Mercury 

 poisonous, although not in such a degree as is usually believed, 

 much less so than some of its chemical compounds, which are amom 

 the most poisonous substances known. Caution is therefore nee 

 sary in handling mercury, that none of it be taken into the mou< 

 or swallowed ; to touch mercury with the hand is harmless, bi 

 even this should not be done without some definite purpose, for the 

 mercury becomes impure by the oily and watery exhalations of the 

 skin. Mercury must not be heated, for it combines in that case 

 with oxygen, one of the constituents of our atmosphere, and 

 thereby not only rendered useless for physical purposes, but becoi 

 also much more poisonous. 



Mercury is somewhat expensive, but one kilogramme is absolul 

 necessary for our purposes ; l k s r measures only 73 CC '5, for l cc 

 13s r> 6, and it is certainly much better to be able to purchase a lat 

 quantity. In any case care is necessary not to lose any of it, for ii 

 mobility and great weight render it rather difficult to handle inei 

 without losing some of it. If mercury falls from a height^ sue 

 as that of a table, upon the floor, it becomes separated into globules 

 so minute, that generally only a very small portion can be recover 

 It is therefore advisable to make all experiments in which mercui 

 is employed upon a tray, consisting of a flat board, surrounded on 

 sides by a rim about 3 or 4 cm high. If the joints between the 

 and the board are not perfectly tight, or become somewhat detach( 

 in course of time by the drying of the wood, the chinks should 

 filled up with putty, or paper should be glued over them. Inste 

 of a tray of wood, one of stout pasteboard may be used ; to rend( 

 it strong enough the joints at the corners must be glued over wii 

 paper inside, and with bands of 'linen, outside ; the glue used for 

 linen should be rather thick. 



The free surface of a liquid is a horizontal plane, bi 

 where the liquid is in contact with the sides of th< 

 vessel or any other solid body, the surface of the liquid 

 becomes curved, and the form of the curve differs 

 according as the solid is or is not moistened by the 



