SOLIDS, LIQUIDS, AND GASES. 299 



broke off and fell into the lake, on the surface of which I saw the 

 result, in the form of several floating icebergs that had previously 

 fallen. 



Something like this, on a small scale, may be seen at home on the 

 edge of a house roof, on which there has been an accumulation of 

 snow ; but, in this case, it is rather sliding than flowing that has 

 made the cornice ; but its down-bending is a result of viscosity. 



These and a multitude of other facts that might be stated, many of 

 which will occur to the reader, prove clearly enough that the solid 

 and liquid states of matter are not distinctly and broadly separable, 

 but are connected by an intermediate condition of viscosity. 



We now come to the question whether there is any similar conti- 

 nuity between liquids and gases. Ordinary experience decidedly sug- 

 gests a negative answer. We can point to nothing within easy reach 

 that has the properties of a liquid and gaseous half -end-half ; that 

 stands between gases and liquids as pitch and treacle stand between 

 solids and liquids. 



Some, perhaps, may suggest that cloud-matter London fog, for 

 example is in such an intermediate state. This, however, is not the 

 case. White country fog, ordinary clouds, or the so-called " steam" 

 that is seen assuming cloud forms as it issues from the spout of a 

 tea-kettle or funnel of a locomotive, consists of minute particles of 

 water suspended in air, as solid particles of dust are also suspended. 

 It has been called " vesicular vapor," on the supposition that it has 

 the form of minute vesicles, like soap-bubbles on a very small scale, 

 but this hypothesis remains unproven. London fog consists of 

 similar particles, varnished with a delicate film of coal-tar, and inter- 

 sprinkled with particles of soot. 



In order to clearly comprehend the above-stated question, we must 

 define the difference between liquids and gases. In the first place, 

 they are both fluids, as already agreed. What, then, is the essential 

 difference between liquid fluidity and gaseous fluidity? The expert 

 in molecular mathematics, discoursing to his kinematical brethren, 

 would produce a tremendous reply to this question. He would de- 

 scribe the oscillations, gyrations, collisions, mean free paths, and 

 mutual obstructions of atoms and molecules, and, by the aid of a 

 maddening array of symbols, arrive at the conclusion that gases, 

 unless restrained, expand of their own accord, while liquids retain 

 definite limits or dimensions. 



The matter-of-fact experimentalist demonstrates the same by 

 methods that are easily understood by anybody. I shall, therefore, 

 both for my own sake and my readers',, describe some of the latter. 



In the first place, we all see plainly that liquids have a surface, i.e., 

 a well-defined boundary, and also that gases, unless inclosed, have 

 not. But as this may be due to the invisibility of the gas, we must 

 question it further. The air we breathe may be taken as a type of 

 gases, as water may of liquids. It has weight, as we may prove by 

 weighing a bottle full of air, then pumping out the contents, weigh- 

 ing the empty bottle, and noting the difference. 



Having weight, it presses toward the earth, and is squeezed by all 

 that rests above it ; thus the air around us is constrained air. It is 

 very compressible, and is accordingly compressed by the weight of all 

 the air above it. 



This being understood, let us take a bottle full of water and 



