NATURAL HISTORY OF THE DIATOMACE^E. 427 



makers of lenses for microscopes, and certain species have been selected 

 and accepted as &quot;test objects,&quot; by means of which the power of lenses 

 to show their structure has been made evident. But as this has become 

 a special department of microscopy we will pass it by, at the present 

 time, with this mere allusion. These beautiful markings are, for the 

 most part, hexagonal, that is to say, six-sided, or, at least, they are of 

 forms derived from the hexagon, that being the form most economical of 

 space under the circumstances. And it can be readily understood how 

 this has come about, if we consider the matter after the following man 

 ner : It is well known that matter, by which is meant solid or semi-solid 

 substance of any kind whatever, if left to itself uninfluenced by any out 

 side force, as gravity and the like, will assume the globular form. Thus 

 we find that the drop of water, of oil, or of metallic quicksilver, is round 

 or spherical, or nearly so. So we can understand that the silica deposited 

 as skeleton by the diatom would assume the spherical form. Likewise, 

 the minute markings or granulations made up by those siliceous particles 

 would, in a like manner, be at least spheroidal, be they elevations, as it is 

 assumed by some observers, or depressions, as is thought to be the case 

 by the majority of students. Now, supposing them to have a circular 

 outline when they are far apart, if they are made to approach each other 

 closer and closer until they touch, they, at last, by mutual pressure, will 

 present a six-sided outline. That such would be the case may be proved, 

 experimentally, in a very simple and, at the same time, elegant manner. 

 Let a mass of soap and water be placed in a bowl, and a pipe like a 

 straw be thrust down into it. Then if air be blown through the tube, 

 keeping the end opposite to that placed in the mouth in contact with the 

 bottom of the bowl, after a time the vessel will become filled with soap- 

 bubbles all of about one size, and which we know, if they had been 

 formed separately, would each have been perfectly spherical, as, in fact, 

 can be seen on the top where those which are outermost present an outer 

 limiting surface which is part of a sphere. But we see plainly that these 

 little globes have pressed upon each other to such an extent that they 

 have lost their spherical outline, and have sides which are more or less 

 plane. If, now, a plate of glass be pressed down upon the accumulation 

 of bubbles in the bowl, many of them will be cut in two in such a way 

 that we may see through the glass that their section is an almost regular 



