experience will soon enable the dyer to form a correct judgment of the
difference in strength between two samples of dye-stuff.

The colorimetric test is based on the principle that the colour (p. 217)
of a solution of dye-stuff is proportionate to its strength. Two white
glass tubes, equal in diameter, are taken; solutions of the
dye-stuffs, 0.5 gramme in 100 c.c. of water, are prepared, care being
taken that the solution is complete. 5 c.c. of one of these solutions
is taken and placed in one of the glass tubes, and 5 c.c. of the other
solution is placed in the other glass tube, 25 c.c. of water is now
added to each tube and then the colour of the diluted liquids is
compared by looking through in a good light. That sample which gives
the deepest solution is the strongest in colouring power. By diluting
the strongest solution with water until it is of the same depth of
colour as the weakest, it may be assumed that the length of the
columns of liquid in the two tubes is in proportion to the relative
strength of the two samples. Thus if in one tube there are 30
centimetres of liquid and in the other 25 centimetres, then the
relative strength is as 30 to 25, and if the first is taken as the
standard at 100 a proportion sum may be worked out as follows:--

30: 25 :: 100 : 83.3;

that is, the weakest sample has only 83.3 per cent. of the strength of
the strongest sample.

CHAPTER IX. (p. 218)

TESTING OF THE COLOUR OF DYED FABRICS.

It is frequently desirable that dyers should be able to ascertain with
some degree of accuracy what dyes have been used to dye any particular
sample of dyed cloth that has been offered to them to match. In these
days of the thousand-and-one different dyes that are known it is by no
means an easy thing to do, and when, as is most often the case, two or
three dye-stuffs have been used in the production of a shade, the
difficulty is materially increased.

The only available method is to try the effect of various acid and
alkaline reagents on the sample, noting whether any change of colour
occurs, and judging accordingly. It would be a good thing for dyers to
accustom themselves to test the dyeings they do and so accumulate a
fund of practical experience which will stand them in good stead
whenever they have occasion to examine a dyed pattern of unknown
origin.

The limits of this book do not permit of there being given a ser