20 BULLETIN 471, U. S. DEPARTMENT OF AGRICULTURE. 
molds and bacteria are not injurious to health, while others may 
give rise to more or less serious illness. The best way to prevent 
the infection of eggs by molds and bacteria is to produce infertile 
eggs and to keep them clean and dry from the time they are laid 
until they are finally used. The coating of mucilaginous material 
on the outside of the egg and the very thin membrane, or " skin," on 
the inside protect the egg from the invasion of molds and bacteria. 
The shell coating is easily removed by water or by rubbing. Also as 
eggs grow old the natural protection of the egg becomes less effective. 
Therefore, deterioration due to molds and bacteria is more likely to 
occur if the egg is washed or the shell becomes moist by being kept in 
a damp place. If eggs are dirty enough to wash it is better to do so 
and use them at once than to " put them down." Neglected and 
stolen nests (usually dirty and so sources of bacteria, etc.) are prob- 
ably the chief cause of molded or rotten eggs, and so hens should 
always have clean, airy nests. Housekeepers who are careful will 
not choose dirtj^ eggs. If for any reason they must be used they 
should be carefully washed and wiped. 
These causes of spoiling are important matters for the house- 
keeper, since they have a bearing on supply and market price. It is 
to the housekeeper's interest to have only eggs produced under good 
conditions brought to market, because this means less loss in handling 
and an increased supply of good eggs, which in turn, affects prices 
favorably. 
As a means of securing a supply of fresh eggs for table purposes, 
the purchase of eggs direct from the producer by parcel post is worth 
consideration by the housekeeper. 
Since eggs vary more or less in size and weight, though very little 
in composition (see p. 6), it follows that they vary in the amount 
of food material which they supply. For this reason housekeepers 
who have a choice very commonly like to select large eggs. That 
size has an effect on the cost of eggs when they are purchased for a 
uniform price per dozen is evident when one considers that a dozen 
pullets' eggs, weighing 18 ounces, at 25 cents per dozen, supplies 
eggs at 22.9 cents per pound, while hens' eggs, weighing 24 ounces 
at the same price per pound would supply eggs at 16f cents per 
pound. Such differences are especially worth considering when eggs 
are used in quantity. There are few exact household standards for 
cookery, and using eggs as they come, by number rather than by 
weight, would probably not imply important variations. However, 
it is well known that when a recipe calls for 4 eggs one may use 
instead of 4 of average size either 3 large or 5 small ones. 
For reasons of economy as well as accuracy in cookery there is a 
growing tendency to use eggs by weight or measure instead of by 
