Aids to Microscopic Inquiry. 77 



a vessel containing this acid will keep a glass case containing 

 bright steel goods qnite dry. If damp gets in, the acid seizes 

 upon it, and the goods are protected from its effects. It is 

 not safe to leave an open vessel of sulphuric acid in a damp 

 cupboard for any length of time, for the moisture will con- 

 dense and at length cause it to overflow. 



It is true that in these, and many similar cases, we have 

 something more than surface action, inasmuch as the whole 

 mass of chloride of calcium, and the whole mass of sulphuric 

 acid, becomes gradually affected by the moisture which the 

 surface particles absorb, and then hand on in due proportion 

 to their neighbours. The surface action is, however, not less 

 real and important because other actions follow it in due 

 succession. 



A still more remarkable surface-action is manifested by 

 the metal platinum. If a strip is perfectly clean, it will so 

 strongly attract a mixture of oxygen and hydrogen gases as 

 to make them unite and form water. The heat given out in 

 the process of combination makes the metal red hot. Surface- 

 action is proportionate to the quantity of surface engaged, 

 Thus platinum in a minute state of division, constituting what 

 is termed " platinum sponge," will instantly ignite a jet of 

 hydrogen thrown upon it in common air ; and many metals 

 take fire in contact with air, if they have previously been 

 reduced to so fine a powder as to cause an enormous and con- 

 tiguous surface to be simultaneously exposed. 



The lungs of animals are constructed so that large and 

 contiguous surfaces of animal tissues shall be simultaneously 

 exposed to the action of air. Gills do analogous work, and 

 thus make the air that is found in water serve the purposes 

 of life. Minute plants and infusorial animals do an immense 

 quantity of work, because in the aggregate they bring a very 

 extensive surface in contact with the fluids in which they grow. 



If we wish to expose any substance to the most complete 

 action some other substance can exert upon it, we endeavour 

 to attack it at once on all sides. A lump of sugar would 

 dissolve in the course of time if one point of it was kept in 

 contact With water ; but we dissolve it with far greater rapidity 

 if we throw it into water, and let the fluid assail it all round 

 and in every nook. This proposition is perfectly obvious ; but 

 the important consequences that flow from it are often over- 

 looked. Suppose we take a flat piece of glass and bend it 

 into a tube. If we put water in that tube it will be exposed 

 to surface action all round. Suppose we reduce the diameter 

 of the tube to the dimensions of a hair. It will hold less fluid, 

 but every particle of what it does hold will be surrounded by 

 a glass surface which will be close to it. Under such circum- 



