328 Frederick Guthrie on certain 
stance to confront it, it rolls along (for that is the motion of a 
slab however large) and forms a " level," which requires a very 
steady support to avoid the notion that its motion is affected 
by the gravity of the observer. 
50 cub. centim. of glycerine at 14° C. has an extreme radius 
of 59 millim. 
7= 50,000 
' 3-1416 x(58) 2; 
thickness = 4-731 millim. 
§ 20. Accordingly, taking the slab thickness of water as 
unity, we may begin a table which will at some future time 
assuredly be extended. 
Specific Slab-thickness (at 14° C). 
Water . = 1-0000, 
Glycerine =0-8602, " 
Mercury =0-5906. 
These numbers may be, with instruction, considered in re- 
ference to the numbers in table vii. which concern the drop 
sizes of the same three liquids in the 'Proceedings of the Royal 
Society,' 1864, p. 17 [" Recess "]. It will, I have no doubt, 
appear that in all cases the greater the drop-size the greater the 
slab-thickness. Water will, no doubt, again assert its singu- 
larity and exhibit the greatest slab-thickness. 
§ 21. Restrained as slabs are in their form by skin-tension 
as well as cohesion, it is found that the addition of a liquid 
which diminishes the former diminishes also the slab-thick- 
ness. Taking 25 cub. centim. of water at 14° C, a slab 
was formed having 38 millim. corrected radius. This gives 
a thickness of 5*51 millim. Such a slab is unchanged if 
touched in the middle by a drop of glycerine. But on touch- 
ing it with " glacial " acetic acid, it instantly acquires a cor- 
rected radius of 44 millim., or thickness of 4*16 millim. This 
means a diminution in thickness of very nearly 25 per cent., 
or one quarter. The question therefore presented itself, What 
is the slab-thickness of " glacial " acetic acid ? 
I reserve the results of my experiments in the direction of 
the relationship between the liquids and the alteration of skin- 
tension. 
§ 22. The mercury slab, like the water slab, has what vir- 
tually amounts to a skin; and it became interesting to exa- 
mine the conditions of this skin or region of surface-tension. 
If lycopodium be strewn upon the surface of a mercury slab, and 
a little tin, zinc, or lead, or amalgam of these metals, be made 
to touch the slab in the middle, no noticeable disturbance 
takes place. But if such a slab be touched by an amalgam 
