B.—CHEMISTRY. 65 
thereto. The first step was their separation into ‘single’ and ‘double’ 
proof, a rough and inconclusive one, but accounting for the use of a term 
still recognised as that on which the full statutory rate of duty is leviable. 
For charging Revenue the gallon was first taken as a measure in 1825, 
but definite alcoholic strength was not introduced as a basis until 1860, 
under a treaty with France, while a little later, in 1862, Parliament dis- 
tinguished between wines above and below 26 degrees of proof spirit, this 
figure being raised in 1886 to 30 degrees. 
The want of some accurate method of test had been felt, and it is 
interesting to follow the gropings after a method for recognising a standard 
strength of alcohol. Thus observations on the surface tension of spirits 
were employed, for Postlethwaite in 1751 described as a mark of their 
being up to proof the length of time elapsing before bubbles disappear 
from the surface of the liquid contained in a glass tube which had been 
‘shaken, but as he believed this method may be falsified, he recommended 
for more accurate work ‘ the essay instrument, or hydrostatical balance,’ 
although for business men it would be sufticient to burn a measured quantity 
of the spirit in a metal cylindrical vessel immersed in cold water, and 
measure the remainder, which should be equal to half the original volume, 
if the spirits were proof. Although ‘ Boyle’s bubble’ had been described 
in 1675, and Moncony’s areometer in 1679, the first instrument generally 
adopted by the Revenue in 1730 was the hydrometer of Clarke, legalised 
in 1787. It is complicated, however, and its temperature correction by 
* weather weights ’ was unsatisfactory, so that Parliament gave instructions 
for “ proper experiments to be made.’ 
At the request of the Government to the President of the Royal 
Society, Sir Charles Blagden (Secretary) and one of the clerks, Mr. George 
Gilpin, undertook to make experiments on the specific gravity of alcohol 
and water in varying proportion. These experiments, conducted with 
_ exemplary care and ability, were reported to the Royal Society in 1790, 
1792, and 1794, and formed the basis for the tables of Sikes, whose hydro- 
meter became the sole legal instrument in 1818, and is still in use. These 
tables remained legal for nearly a hundred years, but in 1916 were replaced 
by a new and extended set, prepared under the supervision of Sir Edward 
Thorpe at the Government Laboratory, whence also in the same year were 
_ issued comprehensive tables of spirit strengths for use with pyknometers, 
as these had shortly before been legalised for alternative use in the deter- 
mination of alcohol. Both of these sets of tables were founded on the 
definition of proof spirit contained in the Act of George IT., which is, that 
_ spirit which at the temperature of 51° Fahr. weighs exactly 12-13 parts of 
an equal measure of distilled water. In other words, it contains 49°28 
parts by weight of pure alcohol and 50°72 parts by weight of distilled 
water. 
As these tables refer only to alcohol-water mixtures, all disturbing 
_ substances must be removed before the strength of liquids is determined 
i by hydrometer or pyknometer. The methods of freeing spirit in com- 
mercial articles from everything but water were investigated and laid 
% down by the Government Laboratory in 1903 by Thorpe and Holmes. 
From the point of view of trade it is highly important to have free use 
ie pot ethyl alcohol, while from that of the Revenue it is essential to prevent 
_ the use of such duty- -free spirit as a beverage. The most effective means 
1924 F 
