12 ANATOMY FOR NURSES [Chap. 1 



Dialysis. — In this process a permeable membrane is selected 

 which will admit of the passage of a substance in solution. For 

 example, if a tumbler is completely divided vertically into two 

 compartments by a moist piece of membrane and a water solution 

 of common salt is placed in one compartment and a water solution 

 of sugar in the other, it will be found after a time that some of the 

 salt has passed into the solution of sugar, and vice versa some of 

 the sugar into the salt solution. Such an interchange is said to 

 be due to dialysis, and if the process is allowed to go on for some 

 hours, the same proportion of salt and sugar will be found in the 

 solution on each side of the dividing membrane. (See page 154.) 



Emulsion. — When a substance scatters evenly throughout 

 a liquid in such a finely divided form that it cannot be separated 

 from the liquid by filtration, but has a tendency to separate like 

 matter in suspension, the system is spoken of as an emulsion. 

 This separation may take place in a brief time, or it may require 

 a week, or month, or even a year. Oil shaken thoroughly in 

 water furnishes an example. 



UNIT FOR MEASURING HEAT 



Inasmuch as heat is a form of energj'', it is not as simple an under- 

 taking to speak of it in comparative terms as in the case of matter. 

 It is easy for one to visualize five quarts of milk as five times a 

 certain volume that has been taken as a standard and called one 

 quart. So it is also with weights and distances, and the measure- 

 ment of weights, distances, and volumes is a necessary part of our 

 experience. It is quite as urgent that there should be a basis for 

 the comparison of energy values as well as for matter values, else 

 the coal dealer could not place the value of his coal on a basis of 

 its heat-producing qualities, nor could the dietitian plan the meals 

 on a basis of the ultimate energy-producing qualities to the individ- 

 ual. We are only conscious of energy as a result of the effect it 

 produces, consequently, if it is to be measured, it must be on this 

 basis. The units most used for measuring heat are the small 

 calorie and the large calorie. 



Small calorie. — The small calorie (cal.) is the amount of heat 

 that is necessary to raise one gram of water through one degree 

 centigrade. 



In the complete combustion of 12 grams of carbon, 94,300 cal. 



