38 



HA RD WICKE'S SCIENCE- G OSSIP. 



that such fibres take the dye more readily, and when 

 -dyed the colour is more permanent. To make use of 

 these important results, and further for the purpose 

 of scouring all grease from the fibres, cotton is usually 

 soaked in caustic soda before it is dyed. Sulphuric 

 acid dissolves cellulose, membrane, and everything. 

 Indeed, all acids, when in weak solution, render the 

 cotton weak and brittle ; some entirely destroy the 

 fibre. 



In the attempt to soak raw cotton in water, it 

 will have been noticed how repulsive it is to water. 

 This is caused by the outer membrane of the fibre, 

 which is really vegetable wax, called by some, cotton 

 wax. It is the same substance as the "bloom" 

 found on some leaves and fruit, and is composed of 



hair. It is similar to that found in the cells of 

 dicotyledonous plants and trees, and is deposited on 

 the inner side of the cell-wall in layers. These layers, 

 or concentric rings, may sometimes be seen in a cross- 

 section of the hair, highly magnified, though the lami- 

 nated form is somewhat difficult to make out. It 

 shows us, however, that the depositing of cellulose is 

 not regular and continuous, but thicker in some 

 places than others — similar to the way in which the 

 rings are formed in the trunk of a tree. 



The existence of spiral fibre in the cotton hair, 

 seems to be the source of its great strength. It has 

 been found that a fibre of a good class of American 

 cotton will resist a strain of 145 grains before it 

 breaks. Now the maximum or theoretical strength 



Fig. 35. — Cotton-Fibre, compared with wool, &c. X 400. (See page 39.) 



the same constituents, and in nearly the same pro- 

 portion as cerosine, a wax obtained from the leaves 

 of the sugar-cane. In bleaching, this waxy covering 

 is dissolved by the hot bleaching liquids, which 

 also remove a fatty acid along with the wax. The 

 loss in weight sustained by bleaching is generally 

 about five per cent, so that if we were to have 

 bleached 100 lbs. of grey calico, the weight after 

 bleaching would be only 95 lbs. The cotton wax, 

 however, only forms a small portion of this loss, as it 

 only constitutes two per cent, of the whole of the 

 fibre. We may describe it as a varnish on the cotton 

 hair, which is impermeable and insoluble in the 

 copper solution previously referred to. 



The cellulose forms about 87 per cent, of the cotton 



of yarn would, of course, be the aggregate strength of 

 the fibres which compose it. But it is found in 

 practice that we cannot get anything near this strength 

 out of yarn ; in fact, the actual is but a fifth of the 

 theoretical strength. The number of fibres in a cross- 

 section varies as to the thickness of the yarn. In 32's* 

 twist, made out of American cotton, there are about 

 140, and in 50's yarn, about ninety in a cross-section. 

 Of course, not all these get twisted in the thread. 

 Many hang out from it, and this explains the great 

 loss of strength. Cannot some improvement be 

 made ? This is the question which is engaging the 



* The numbers or counts are given to denote the thickness of 

 the yarn. They indicate the numbei of hanks, each 840 yards, 

 in 1 lb. of yarn. 



