38 General Biology 



subject of osmosis, diffusion, and capillary attraction must be under- 

 stood before such an experiment can be intelligently applied. 



The place where parasites are most likely to lodge, is largely deter- 

 mined by the rapidity, direction, and pressure of the blood-stream. 



Hydrometers and urinometers for testing liquid densities are built 

 on the principles just enumerated. 



Air is a gas, and as such comes under the laws governing gaseous 

 pressure and gaseous diffusion. When it is remembered that the whole 

 process of life is snuffed out when the breathing apparatus ceases to 

 work properly, it will be seen that the aeration of the blood to keep it 

 red and healthy, the working of the lungs under normal and abnormal 

 conditions (the latter in chest puncture), the being overcome by gas, 

 externally or internally, as well as the changes in breathing at different 

 altitudes and at different depths (as on mountain-tops and in subma- 

 rines) ; all these can only be understood and helped by a thorough study 

 of the laws *and principles applying to gasses. 



The principle of the force-pump makes the pumping of the heart 

 and the one-way valves in the heart and veins understandable. 



All food eaten can only be reduced to blood by a burning process, 

 called oxidation. Unless the principles governing heat are understood, 

 the processes of digestion and the consequent abnormality — indigestion 

 — must go on unremedied. 



The principles of ventilation in the home, office, work-shop, or sick 

 room, make for health or disease, just as one applies them or leaves them 

 unapplied. A window opened at the top warms the incoming air before 

 it strikes the patient. The knowledge that warm air ascends and cold 

 air descends suggests that a heating plant must be placed in the base- 

 ment and a cooling plant in the attic. The principle of evaporation 

 explains how outpourings of the sweat glands, by being drawn off 

 rapidly into the surrounding atmosphere, make it possible for warm- 

 blooded animals to retain an even temperature, regardless of varying 

 environment. 



In "chills" the body really produces more heat than ordinarily, but 

 it is the heat-regulating apparatus which is out of order. 



Thermometers and hygrometers are measuring instruments by 

 which we note the amount of heat and moisture respectively in the 

 atmosphere. They are, of course, simply applications of laws learned 

 in physics. 



Then, too, boiling and sterilizing make food, which is normally un- 

 palatable and sometimes even injurious, palatable and non-injurious. It 

 is sterilization also which makes antiseptic surgery possible. 



The laws governing liquids under varying conditions of heat give 

 us the basis for understanding evaporation, condensation, distillation, 

 conductivity, convection, radiation, and even plumbing and heating. It 

 explains why germs can be killed at a much lower temperature when 

 there is moisture in the air (steam) than otherwise. In fact, a human 



