8 1 4 THE POPULAR SCIENCE MONTHLY. 



this first step was soon followed by another, even more important 

 step, when Van der Waals also a Dutch chemist still more 

 effectively bridged over the gap between the gaseous and liquid 

 condition of matter. He studied that state of a gas when, under 

 an increasing pressure and a decreasing temperature, it becomes 

 a liquid ; and he found a mathematical expression (an equation) 

 which very approximately represents the mutual dependence be- 

 tween the volume occupied by the gas under a given pressure, its 

 temperature, the volume occupied by its particles, and their 

 mutual pressure. He thus expressed in a more comprehensive 

 way how, in proportion as the lengths of the paths of its particles 

 decrease, a gas becomes a liquid.* 



The long-since suspected continuity between the gaseous and 

 liquid states of matter was thus demonstrated once more, and 

 rendered easy to investigate ; and the importance of these conclu- 

 sions was still more enhanced by Clausius, when he demonstrated 

 that a slight alteration of Yan der Waals's equation makes it also 

 represent the absorption or dissipation of heat-energy which al- 

 ways takes place when a body passes from the liquid to the gase- 

 ous state, or vice versa. 



And, finally, another step in the same direction was made by 

 the French physicist, Raoult. "We all know that if some table- 

 salt, or saltpeter, or some other salt, be added to water, the water 

 may be cooled below zero without freezing. Its freezing tem- 

 perature is lowered. JSTow, Raoult studied the lowering of this 

 temperature caused in water and other liquids by the addition of 

 various amounts of various salts, and he came to a most remark- 

 able result. It appeared that, whatever the nature of the dissolved 

 salt may be, the freezing temperature of a solution will always 

 be lowered by the same amount (nearly six tenths of a degree) if 

 we add one molecule of the dissolved body to each hundred mole- 

 cules of the solvent, f Thus, again, a purely physical fact, such 

 as freezing, proves to be dependent upon a purely chemical fact 

 the molecular weights of the solvent and the dissolved body ; and 

 this physical law is so general that it has become a very accurate 

 means for determining such chemical data as molecular weights. 

 Chemistry and physics appear again so closely interwoven that 

 there is really no means of separating them. 



* See the interesting discussions which took place upon this subject in the Physical So- 

 ciety, in October and November last. 



f Thus, if table-salt be used, the weight of its molecule (compared with a molecule of 

 hydrogen) is 58i ; while the weight of a molecule of water (also compared with hydrogen) 

 is 18. So that, if we add 58A ounces of table-salt to each 1,800 ounces of water, we shall 

 lower its freezing temperature by - 62 of the centigrade scale. The same result will be 

 obtained if we take 141 ounces of potassium chloride, or 101 ounces of saltpeter, to the 

 same amount of water. 



