408 CHEMISTRY. 



nous matter than wheat, and therefore would be more nutri- 

 tious were they not deficient in a salt of lime, the phosphate. 

 There is considerable lime supplied to plants and animals 

 from water in the forms of the carbonate and the sulphate, 

 these salts being present in all hard water. The thickness 

 of the shells of aquatic mollusks depends very much upon 

 the amount of carbonate of lime in the water. Those which 

 live in the sea have as much as they need, and their shells 

 have considerable thickness ; but those which are found in 

 fresh-water lakes, where there is but little lime, have thin 

 shells. There are some lakes, however, where, from local 

 causes, the water is greatly impregnated with calcareous 

 matter, and the mollusks that inhabit them have shells of 

 uncommon thickness. Hens require more lime than usual 

 when they are laying eggs, and they therefore instinctive- 

 ly at such times eat chalk, mortar, or any substance they 

 can find which contains carbonate of lime. If they are 

 shut up where they can not obtain this they lay eggs 

 without shells, and if they obtain it sparingly the shells 

 are thin. 



579. Iron. This metal is absolutely essential to the 

 blood, and is present in all the pigments of the body, in 

 the bile, and in various tissues, especially the hair. The 

 quantity of iron in the blood is very small, it being only 

 about the one four-hundredth part of its solid matter. It 

 varies in different persons, being greater in the sanguineous 

 than the lymphatic, in the well-fed than those who live on 

 a poor diet. It is found more or less in most articles of 

 food. It is in the yolk of eggs and in milk, as well as 

 in animal flesh. It is present in most of the vegetable 

 substances used as food by man, such as potatoes, cabbage, 

 pease, mustard, etc. We have said that iron is essential to 

 the blood. It is there not so much to be supplied to the 

 tissues as to execute certain offices in the blood itself. 



