Human Physiology. 89 







that we see in the world around us. Oxygen and nitrogen, 

 mingled together, make the air we breathe. Oxygen and hydro- 

 gen, chemically combined, make the water we drink, and which 

 forms four-fifths of all vegetable and animal bodies. Oxygen 

 and metals combine to form the earth on which we live, and 

 from which we draw our subsistence. Carbon unites with 

 oxygen and hydrogen to form vegetables and then animals; it 

 is the basis of all life on the earth, and the vast stores of fuel 

 laid up for man in remote ages. 



The same ultimate elements or atoms, combining in slightly 

 varying proportions, or with some unknown variation of form, 

 produce a wonderful variety of substances. The same elements 

 found in a dunghill enter into the composition of multitudes of 

 delicious fruits and odorous flowers. The most stinking gas 

 and otto of roses are composed of the same elements in the 

 same proportions. With very slight changes in proportions we 

 have sugar, starch, celulose, alcohol, lactic acid. The foetid 

 excretions of animals become the food of the most delicate and 

 beautiful products of the vegetable kingdom. From the same 

 food we have milk, wax, honey, poison; from the blood of 

 animals come sweet milk, salt tears, bitter gall, odorous musk, 

 and the deadly venom of the rattlesnake and cobra. 



Two collections of atoms separately invisible, become visible 

 when joined together. If I write with a clear solution of tannic 

 acid, the letters cannot be seen. Wet them with an equally 

 colourless solution of sulphate of iron, and they come out black. 

 So cloths are dyed by being dipped successively into two 

 colourless liquids. 



Lime water is perfectly liquid and transparent. Take a tube 

 and blow in it. and you have an opaque solid, which first 

 clouds the water, and then settles to the bottom. Transparent 

 glass, if stirred when cooling, becomes opaque. In transparent 

 bodies the atoms seem to arrange themselves in straight lines, 

 that the vibrations of light can pass through them in every 



