NATURAL PHILOSOPHY. 133 



commencement of the experiment, and to heat it in a bath of 

 paraffine or oil of vitriol. 



" The distinction between a gas and vapor has hitherto been 

 founded on principles which are altogether arbitrary. Ether in 

 the state of gas is called a vapor, while sulphurous acid in the 

 same state is called a gas ; yet they are both vapors, the one derived 

 from a liquid boiling at 35, the other from a liquid boiling at 10. 

 The distinction is thus determined by the trivial condition of the 

 boiling-point of the liquid, under the ordinary temperature of the 

 atmosphere. Such a distinction may have some advantages for 

 practical reference, but it has no scientific value. The critical 

 point of temperature affords a criterion for distinguishing a vapor 

 from a gas, if it be considered important to maintain this dis- 

 tinction at all. Many of the properties of vapors depend on the 

 gas and liquid being present in contact with one another; and 

 this, we have seen, can only occur at temperatures below the 

 critical point. We may accordingly define a vapor to be a gas at 

 any temperature under its critical point. According to this definition 

 a vapor may, by pressure alone, be changed into a liquid, and may 

 therefore exist in presence of its own liquid; while a gas cannot 

 be liquefied by pressure, that is, so changed b}^ pressure as 

 to become a visible liquid distinguished by a surface of demarcation 

 from the gas. If this definition be accepted, carbonic acid 

 will be a vapor below 31, a gas above that temperature ; ether 

 a vapor below 200, a gas above that temperature. 



" We have seen that the gaseous and liquid states are only dif- 

 ferent stages of the same condition of matter, and are capable of 

 passing into one another by a process of continuous change. A 

 problem of far greater difficulty yet remains to be solved, the 

 possible continuity of the liquid and solid states of matter. But 

 this must be a subject of future investigation; and for the present 

 I will not venture to go be} T oud the conclusion, I have already 

 drawn from direct experiment, that the gaseous and liquid forms 

 of matter may be transformed into one another by a series of con- 

 tinuous and unbroken changes." 



SUPERSATUKATION. 



Mr. J. G. Grenfell has described several interesting experi- 

 ments, bearing on the theory of supersaturation. Professor 

 Tomlinson supposes that a supersaturated solution adheres as a 

 whole to a chemically clean surface, while to a chemically unclean 

 surface the salt or gas adheres, while the liquid does not, thereby 

 liberating the former. Grease is considered to be a cause of 

 uncleanness. If, now, a greasy surface can be rendered inactive, 

 either grease is not a cause of uncleanness, or unclean surfaces 

 are not necessarily inactive. A number of experiments were 

 tried, in which a drop of melted fat was allowed to fall on a super- 

 saturated solution, without causing solidification even upon agita- 

 tion of the liquid, while a minute crystal of the salt caused instant 

 solidification. Mr. Tomlinson's explanation, then, would seem 

 to be inadequate. M. Gerney believes that only a crystal of the 



