Aids to Microscopic Inquiry. 75 



quickly in their feet which a moment before was extended its 

 full length ; the same results have been obtained with the 

 Anodonta piscinale, and very often I have had to draw in 

 my breath while studying these animals/'' 



I hope some of my readers will take care to prove for 

 themselves the above-recorded fact. I confess I have not 

 found these niollusca very susceptible of sound. 



AIDS TO MICKOSCOPIC INQUIRY. 



II. — Capillaet and Surface Actions. 



Many of the phenomena presented by organized beings ex- 

 emplify the physical actions of surfaces and capillary tubes. 

 If two clean surfaces of substances that do not repel each 

 other are pressed together, they adhere with more or less force. 

 Plates of glass that are perfectly clean, exquisitely smooth, and 

 accurately flat, may be made to stick together so tightly that 

 they cannot be separated without breaking. In like manner 

 plates of steel, if quite clean and worked with sufficient ac- 

 curacy, will cohere, and only submit to separation when great 

 force is applied. The explanation of these facts is that we 

 have artificially reproduced the conditions under which the 

 particles of solids naturally cohere. In order, for example, to 

 form a block of iron it is necessary that the molecules of the 

 metal should be brought very near each other, and that no 

 substance should come between them capable of counteracting 

 their mutual attraction. If, therefore, two surfaces such as 

 we have mentioned fit so close to each other that the particles 

 of one surface come within reach of the molecular attraction 

 exerted by the particles of the other, they will behave as 

 though they had formed part of a primitive undivided mass. 

 If the contact is perfect at all points, we shall really have 

 obtained a firm, solid mass ; but it will often happen that the 

 adhesion is incomplete at some portions of the two surfaces, and 

 then the imperfect union will be broken through on the appli- 

 cation of a force much less than would have sufficed to sever 

 the parts of a block in a normal state. 



If we take an ounce of shot and melt them in a crucible, 

 we obtain a uniform mass of lead. The heat, by reducing the 

 metal to a fluid state, enabled the particles to run together, or, 

 in common language, to come into such close contact as to 

 form one substance. If we take a piece of sheet lead, make 

 its surface perfectly bright and clean, and then press it 

 tightly against another piece similarly prepared, we shall have 



