f detecting adulteration.

Where a great number of samples have to be tested expeditiously, the
Abbe refractometer or the Zeiss butyro-refractometer may be recommended
on account of the ease with which they are manipulated. The most usual
temperature of observations is 60 deg. C.

The _Titre_ or setting point of the fatty acids was devised by Dalican,
and is generally accepted in the commercial valuation of solid fats as a
gauge of firmness, and in the case of tallow has a considerable bearing
on the market value.

One ounce of the fat is melted in a shallow porcelain dish, and 30 c.c.
of a 25 per cent. caustic soda solution added, together with 50 c.c. of
redistilled methylated spirit. The whole is stirred down on the water
bath until a pasty soap is obtained, when another 50 c.c. of methylated
spirit is added, which redissolves the soap, and the whole again stirred
down to a solid soap. This is then dissolved in distilled water, a
slight excess of dilute sulphuric acid added to liberate the fatty
acids, and the whole warmed until the fatty acids form a clear liquid
on the surface. The water beneath the fatty acids is then syphoned off,
more distilled water added to wash out any trace of mineral acid
remaining, and again syphoned off, this process being repeated until the
washings are no longer acid to litmus paper, when the fatty acids are
poured on to a dry filter paper, which is inserted in a funnel resting
on a beaker, and the latter placed on the water-bath, where it is left
until the clear fatty acids have filtered through.

About 10-15 grammes of the pure fatty acids are now transferred to a
test tube, 6" x 1", warmed until molten, and the tube introduced through
a hole in the cork into a flask or wide-mouthed bottle. A very accurate
thermometer, graduated into fifths of a degree Centigrade (previously
standardised), is immersed in the fatty acids, so that the bulb is as
near the centre as possible, and when the fatty acids just begin to
solidify at the bottom of the tube, the thermometer is stirred round
slowly. The mercury will descend, and stirring is continued until it
ceases to fall further, at which point the thermometer is very carefully
observed. It will be found that the temperature will rise rapidly and
finally remain stationary for a short time, after which it will again
begin to drop until the temperature of the room is reached. The maximum
point to which the temperature rises is known as the "tit