January 21, 1897] 
NATURE 
273 
to impacts, between the molecules of the liquid S and 
the molecules of the solid piston Q. We are left abso- 
lutely without theoretical guide as to the resultant force 
due to the impacts of S molecules and D molecules 
striking the other piston, P, and rebounding from it, and 
their attractions upon its molecules ; andas to the numbers 
per unit volume of the S molecules on the two sides of 
MM, except that they are not generally equal. 
No molecular theory can, for sugar or common salt or 
alcohol, dissolved in water, tell us what is the true 
osmotic pressure against a membrane permeable to 
water only, without taking into account laws quite un- 
known to us at present regarding the three sets of mutual 
attractions or repulsions: (1) between the molecules of 
the dissolved substance ; (2) between the molecules of 
water ; (3) between the molecules of the dissolved sub- 
stance and the molecules of water. Hence the well- 
known statement, applying to solutions, Avogadro’s law 
for gases, has manifestly no theoretical foundation at 
present; even though for some solutions other than 
mineral salts dissolved in water, it may be found some- 
what approximately true, while for mineral salts dis- 
solved in water it is wildly far from the truth. The 
subject is full of interest, which is increased, not 
diminished, by eliminating from it fallacious theoretical 
views. Careful consideration of how much we can 
really learn with certainty from theory (of which one 
example is the relation between osmotic pressure and 
vapour pressure at any one temperature) is exceedingly 
valuable in guiding and assisting experimental efforts 
for the increase of knowledge. All chemists and 
physicists who occupy themselves with the “theory of 
solutions,” may well take to heart warnings, and leading 
views, and principles, admirably put before them by 
Fitzgerald in his Helmholtz Memorial Lecture (7vazs- 
actions of the Chemical Society, 1896) of January 1896 
(pages 898-909). KELVIN. 
METHOD FOR MEASURING VAPOUR 
OF LIQUIDS.! 
Apparatus for realising the proposed method is re- 
presented in the accompanying diagram. Two Woolff’s 
bottles, each having a vertical glass tube fitted air- 
tight into one of its necks, contain the liquids the 
difference of whose vapour pressures is to be measured. 
Second necks of the two bottles are connected by a bent 
metal pipe, with a vertical branch for connection with an 
air pump, provided with three stopcocks, as indicated in 
the diagram. Each bottle has a third neck, projecting 
downwards through its bottom, stopped by a glass stop- 
cock which can be opened for the purpose of introducing 
or withdrawing liquid. The upper ends of the glass tubes 
are also connected by short india-rubber junctions witha 
bent metal pipe carrying a vertical branch for connection 
with an air-pump. This vertical branch is provided with 
a metal stopcock, 
To introduce the liquids, bring open vessels containing 
them into such positions below the bottles that the necks 
project downwards into them. Close the glass stopcocks 
of these lower necks, open all the other six stopcocks, and 
produce a slight exhaustion by a few strokes of the air- 
pump. Then, opening the glass stopcocks very slightly, 
allow the desired quantities of the liquids to enter, and 
close them again. They will not be opened again unless 
there is occasion to remove the whole or some part of the 
liquid from either bottle ; and, unless explicitly mentioned, 
will not be included among the stopcocks referred to in 
what follows. It will generally be convenient to make 
PRESSURES 
1 “On a Differential Method for Measuring Differences of Vapour Pres- 
sures of Liquids at One Temperature and at Different Temperatures.” 
(Communicated to the Royal Society of Edinburgh, January 18.) By 
Lord Kelvin, G.C.V.O.1. 
NO. 1421, VOL. 55] 
the quantities of the two liquids introduced such, that they 
stand at as nearly as may be the same levels in the two 
bottles, as indicated in the drawing. 
Operation No. 1.—Close the stopcock on the lower 
passage from the bottles to the air-pump (which, for 
brevity, we shall call the lower air-pump stopcock) ; and, 
with the other five stopcocks all open, work the air-pump 
till the liquid in one of the glass tubes rises to within a 
centimetre of the india-rubber collar round its top. 
> dung Hy 
Jo enmasou mars 
Operation No. 2.—Open the lower air-pump stopcock 
till the liquids fall down the tube, nearly down to hydro- 
static equilibriums in the bottles. Close it again, and 
work the air-pump till the liquid in one of the glass tubes 
rises to within a centimetre of the india-rubber collar. 
Operation No. 3.—Repeat operation No. 2 over and over 
again until you cannot, however long you go on pumping, 
get the liquid in either tube to rise within a centimetre of 
the india-rubber collar. 
Operation No. 4.—Continue Operation No. 3 until the 
