DIMINUTION IN BULK BY MIXTUHE. 105 



indurate it, and make its pores smaller. When highly porous 

 septa are used, through which water will percolate pretty readily by 

 its weight only (such as unglazed stoneware or porcelain vessels, 

 e.g., a terra cotta water cooling caraffe, which is always kept 

 externally moist by the slow passage of the water through the pores, 

 thus causing continual evaporation and consequent cooling), the 

 phenomena become somewhat more complex, as the dissolved 

 substances to some extent pass through uninfluenced by the walls 

 of the passages, as they would through a series of fine-bored pipes ; 

 in such cases certain other peculiarities are noticeable, which will 

 be more conveniently examined in connection with the subject of 

 CapiUarity (Chapter XVII.) 



This selective passage of certain bodies through membranes and 

 not of others, is very closely connected with many processes going 

 on in living bodies ; thus the processes of " secretion " occurring in 

 the human body (such as the separation of urine from blood in the 

 kidney, and the production of tears and perspiration) are actions 

 very similar in their nature to osmotic and dialytic phenomena ; 

 whilst the processes of exhalation of carbon dioxide from the lungs, 

 and inhalation of oxygen from the air, are again closely allied to 

 these actions, as well as those physical processes where gases are 

 dissolved in fluids (Chapter VI). 



Alteration in Bulk on Intermixture. 



When liquids are mixed together it not unfrequently happens 

 that the bulk of the mixture is less than that of the two con- 

 stituents before being intermixed ; just as when a solid is dissolved 

 in water, the bulk of the solution is usually less than the sum of 

 the volumes of the original water and solid before being dissolved. 

 In the act of dissolving a solid, it often happens that cold is 

 developed (Expt. 24) although the reverse happens in certain 

 cases where chemical action takes place, the water partly combining 

 with the substance dissolved, as it does with quicklime in slaking 

 (Expt. 240). When two liquids are mixed together and contraction 

 in volume takes place, a considerable production of heat is often 

 noticeable, even in cases where no chemical action (according to 

 the ordinarily received views) takes place. One of the best illus- 

 trations of this is where sulphuric acid and water are intermixed. 



Expt. 98. Two Half Pints do not always make a Whole 

 Pint. Carefully measure out in two long narrow measuring flasks 

 two exact half pints (or other convenient volumes) of oil of vitriol 

 and water respectively ; pour the water into a large flask or stone- 

 ware jug, and then pour a little of the acid carefully into it ; the 



