38 GENERAL BIOLOGY 



Hydrometers and urinometers for testing liquid densities are built 

 on the principles just mentioned. 



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 whenever 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 

 heights 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 heart and veins understandable. 



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

 called oxidation, so that unless the principles governing heat are under- 

 stood, the processes of digestion and the consequent abnormality indi- 

 gestion 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 lets us know that a heating plant must be placed in the 

 basement and a cooling plant in the attic, while the principle of evapo- 

 ration 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 

 environments. 



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 are able to note the degree of heat and moisture respectively 

 in the atmosphere. They are, of course, simply an application of some 

 of the laws learned in physics. 



Then, too, boiling and sterilizing make food, normally unpalatable 

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

 zation also which makes antiseptic surgery possible. 



The laws governing liquids under varying* conditions of heat give 

 us our 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 

 being can live in a boiling-point temperature if the air is dry, but he 

 cannot live in anywhere near so high a temperature if there is moisture. 



The entire understanding of the working of the ear is a matter of 



