THE BEARING OF OSMOTIC PRESSURE 



79 



gas pressure and osmotic pressure gave us the first scientific 

 theory of dilute solutions. 



IMPORTANCE OF A CORRECT THEORY OF SOLUTIONS 



This raises our old question, of what scientific value is a correct 

 theory of solutions? I wish to dwell a little upon this point on 

 account of its fundamental importance. Van't Hoff showed 

 that the laws of gas pressure apply to the osmotic pressure of 

 solutions. In a word, there is some close connection between 

 the properties of solutions and those of gases. Why was this 

 step so distinct!}^ epoch-making? 



We know matter in three states of aggregation, solid, liquid 

 and gas. Of the first we know comparatively little. We are 

 familiar with the crystallographic forms and the systems in which 

 matter crystallizes. We pass different forms of energy; heat, 

 light, electricity, etc., through matter in the solid state, and observe 

 its conductivity for energy, as it is termed. But after all we 

 know very little about solids. We do not know even the chemi- 

 cal formula of ice or rock salt. The one we say is H2O, but we 

 know that this is#not true. We know that it is (1120) ^ but we 

 do not know the value of x and have no reliable means at present 

 of finding it out. We WTite rock salt NaCI, but we know that it 

 is (NaCl)y, and the y in this case is just as much of an enigma as 

 the X in the case of water. I cite these cases to illustrate how 

 nearly perfect our ignorance of matter in the sohd state is, even 

 at present. We know practically nothing about the laws of soUds. 



When we come to liquids our knowledge is only a little more 

 satisfactory. We do know more about liquids than about solids. 

 We know in many cases their molecular aggregation or, as ' we 

 say, their molecular weights. We have studied many of their 

 physical properties. But with this state of aggregation of matter 

 our knowledge is not only far from complete but, indeed, very 

 unsatisfactory. We kno.v very little about the laws to which 

 matter in the liquid condition conforms. 



\\Tien we come to gases it is fortunately a different story. 

 Here we actually know a great deal. Many of the fundamental 



