58 PRINCIPLES OF CHEMISTRY 



care, because the loss in weight may sometimes proceed from the de- 

 composition of the substance itself, with disengagement of gases or 

 vapour. In making exact weighings the hygroscopic capacity of sub- 

 stances that is, their capacity to absorb moisture must be continually 

 kept in view, as otherwise the weight will be untrue from the presence 

 of moisture. The quantity of moisture absorbed depends on the degree 

 of moisture of the atmosphere (that is, on the tension of the aqueous 

 vapour in it) in which a substance is situated. In an entirely dry 

 atmosphere, or in a vacuum, the hygroscopic water is expelled, being 

 converted into vapour ; therefore, if we have the means of drying yases 

 (or a vacuum) that is, of removing the aqueous vapour from them 

 objects impregnated with water may be entirely dried by placing them 

 in such a desiccated atmosphere. The process is aided by heat, as it 

 increases the tension of the aqueous vapour. Phosphoric anhydride (a 

 white powder), liquid sulphuric acid, solid and porous calcium chloride, 

 or the white powder of ignited copper sulphate are most generally 

 employed in drying gases. They absorb the moisture contained in air 

 and all gases to a considerable, but not unlimited, extent. Phosphoric 

 anhydride and calcium chloride deliquesce, become damp, sulphuric acid 

 changes from an oily thick liquid into a more mobile liquid, and ignited 

 copper sulphate becomes blue ; after which changes these substances 

 partly lose their capacity of holding water, and can, if it be in excess, 

 even give up their water to the atmosphere. We may remark that the 

 order in which these substances are placed above corresponds with the 

 order in which they stand in respect to their capacity for absorbing 

 moisture. Air dried by calcium chloride still contains a certain amount 

 of moisture, which it can give up to sulphuric acid. The most com- 

 plete desiccation takes place with phosphoric anhydride. Water is also 

 removed from many substances by placing them in a basin over a vessel 

 containing a substance absorbing water under a glass bell. 14 The 

 bell, like the receiver of an air pump, should be hermetically closed. 



more rapidly on heating, the moisture given up by the substance in the tube will be 

 carrietl off by the air passing through it. This damp air then passes through a U-shaped 

 tube full of pieces of pumice stone moistened with sulphuric acid, which absorbs all the 

 moisture given off from the substance in the bulb tube. Thus all the water expelled 

 from the substance will collect in the U tube, and so, if this be weighed before and after, 

 the difference will show the quantity of water expelled from the substance. If only water 

 (and not any gases) come over, the increase of the weight of the U tube will be equal to 

 the decrease in the weight of the bulb tube. 



14 Instead of under a, glass bell, drying over sulphuric acid is often carried on in a 

 desiccator composed of a wide-mouthed low glass vessel, closed by a well-fitting ground- 

 glass stopper. Sulphuric acid is poured over the bottom of the desiccator, and the 

 substance to be dried is placed on a glass stand above the acid. A lateral glass tube with 

 a stop-cock is often fused into the desiccator in order to connect it with an air pump, and 

 so allow drying under a diminished pressure, when the moisture evaporates more rapidly. 



