Molecular Constants. 327 
for mercury. But it was found that for the latter liquid a 
sheet of blotting-paper wetted and allowed to dry on a sheet 
of plate-glass gave results identical with those of the paraffin 
surface. The paper surface was used in some of the expe- 
riments. As to the allowance for the meniscus, it is clear 
that this is of less consequence with large slabs than with 
small ones. Indeed, with slabs a few inches in diameter the 
meniscus might be neglected. This was imperfectly shown 
in the case of mercury by adding exactly equal volumes to a 
small slab. After the slab had passed 2 inches in diameter, 
each additional volume produced a " parabolic " increase in 
the diameter. Data derived from this and from the mea- 
surement of an enlarged photograph of the edge gave me 
as a mean 2 millim. to be deducted from the diameter in the 
case of mercury. Assuming it to be the same for water, 
the error incurred, after making this reduction, could not 
in a 6-inch slab be more than -^q of the diameter. This 
would be negligible in the deduced thickness. 
§ 19. I give the following datum for mercury on account 
of the accidental coincidence of the experimental numbers 
with numbers easy of remembrance, excepting as to the tem- 
perature, which is, however, not far from the conventional 
temperature of 60° F. 
100 cub. centim. of mercury at 14° C. has an extreme 
radius of 100 millim. 
, 100,000 
3-1416 x 99* ; 
thickness of mercury slab = 3*248 millim. 
In the case of water it was found so difficult to get a nearly 
circular slab with 100 cub. centim., that only 50 were em- 
ployed. The slab may then be teased into a circular form by 
means of a stick of paraffin covered with lycopodium. 
50 cub. centim. of water at 14° C. has an extreme radius of 
54" 8 millim. 
, 50,000 
3-1416 x (53-8) 2 ' 
thickness =5*50 millim . 
Glycerine* is a beautiful liquid in this respect. It is kept 
off from the paraffin surface by a very faint blush of lycopo- 
dium, and it travels very slowly. It can be got into a circular 
slab more easily than water; but, perhaps on account of its 
capillary action towards its lycopodium props, it is more per- 
sistent in its motion. In fact, unless there be hills of that sub- 
* Commercia], "Price's." 
