252 PROGEESS IN CHEMISTEY. 



and of the disposal of sewage, of our luaimfactures of beer and spirits, 

 of wine growers, and more recently of farmers. All these processes 

 depend upon the action of organisms in producing chemical changes, 

 whether in the tissues of the body, causing or curing disease, or in the 

 production of flavored alchohol from sugar, or in the manufacture of 

 butter and cheese, or in preparing the land for the reception of crops. 

 We also owe to the genius of Van't Hofl' the most important advance 

 of recent times in the region of physical chemistry. It had been 

 observed by M. Raoult, professor at Gi'enoble, that the freezing point 

 of a solvent as a general rule is lowered to the same extent if there be 

 dissolved in it quantities of substances proportional to their molecular 

 weights. Thus supposing 1.80 grams of grape sugar be dissoh^ed in 

 100 grams of water and the solution cooled below 0° C. with constant 

 stirring, ice separates suddenl}' in thin spicules, and the temperature 

 rises to —0.185'^. If 3.42 grams of cane sugar be similar!}- dissolved in 

 100 grams of water the freezing point of the solution is again — 0.185'-'. 

 Now 1.80 and 3.42 are respectively the hundredth part of the molecular 

 weights of grape sugar (CgHjjOg) and cane sugar, CigllggOji. Simi- 

 larly, Raoult found that quantities proportional to molecular weights 

 dissolved in a solvent depress the vapor pressure of that solvent 

 equalh^, or, what comes to the same thing, raise its boiling point b}' 

 an equal number of degrees. But ordinary salts, such as sodium 

 chloride, potassium nitrate, etc., dissolved in water, give too great a 

 depression of the freezing point and too high a boiling point. Next, it 

 had been observed b}" botanists, De Vries, Pfefl'er, and others, who had 

 examined the ascent of sap in plants, that if a vessel of unglazed porce- 

 lain, so treated as to cause a film of cupric ferrocyanide (a slimy red com- 

 pound) to deposit in the pores of its walls', be filled with a weak (aljout 

 1 per cent) solution of sugar or similar substance, and plunged in a vessel 

 of pure water, water entered through the pores. By attaching a manom- 

 eter to the porous vessel the pressui-e exerted by the entering water 

 could be measured. Such pressure was termed " osmotic pressure," 

 referring to the "osmosis," or passage through the walls of the vessel. 

 Such prepared walls are permeable f reeh' to water, but not to sugar 

 or similar bodies. Van't Hofi' pointed out that the total pressure regis- 

 tered is proportional to the amount of substance in solution and that 

 it is proportional to the absolute temperature, and he showed, besides, 

 that the pressure exerted by the sugar molecules is the same as that 

 which would be exerted at the same temperature were an equal num- 

 ber of molecules of hydrogen to occupy the same volume as the sugar 

 solution. This may be expressed by stating that when in dilute solu- 

 tion sugar molecules behave as if they were present in the gaseous 

 state. Here again, however, it was noticed that salts tended to give a 

 higher pressure; it was difficult to construct a semipermeable dia- 

 phragm, however, which would resist the passage of salt molecules, 

 while allowing those of water to pass freely. Lastly, Arrhenius, of 



