In dirty byres in which care is not taken in milking, quite considerable quantities of hairs, pieces of manure, and other filth may get into the milk. Usually the milk is strained by the dairyman, but sometimes this is omitted or carelessly done. To test for dirt, a ribbed glass funnel is useful. Get a piece of the finest muslin about twice the diameter of the funnel, fold over twice, so that it becomes one quarter of its original size; open one of the sections and place in the funnel; pass the milk into this. It will run through quickly and some water may be run into the funnel to clear away the last traces of milk. The filter cloth can then be opened out and any dirt retained will become visible. The apparatus is shown in Fig. 6. [Illustration: TESTING-GLASS FOR EXTRANEOUS MATTER IN MILK. FIG. 6.—A piece of muslin is folded as shown and a measured quantity of milk is passed through the funnel; from the sediment left in the muslin, the percentage of extraneous matter may be arrived at.] If a glass funnel is not available, a very small jelly bag can be made of fine gauze and used in the same way. The washing water should be used in small quantities and directed to concentrating the dirt in the apex of the bag. After washing, the latter can be turned outside in, to permit of readier examination of the dirt. The bag should be well washed in cold water, then boiled and dried, and is then ready for future use. The acidity of milk is a very useful guide to its age. Milk has the curious property of being "amphoteric," _i.e._, it is both slightly acid and slightly alkaline when fresh. As its age increases, however, so does its acidity, and at a rate varying with the temperature and moisture contents of the atmosphere in which it is placed. Old and acid milk is heavily contaminated with bacteria, a proportion of which are likely to be injurious to health. [Illustration: FIG. 7. Lactometer and Test Tube] The simplest method of testing the acidity is to procure a few little books of blue and red litmus test papers, and these can be had from any philosophical instrument maker or laboratory furnisher. The strips of test paper are torn out and dipped in the milk. When the milk is quite fresh it will, owing to its amphoteric condition, change the red litmus paper slightly blue, and the blue litmus paper slightly red. Old milk changes blue litmus paper to a bright red because of its decided acidity. The above tests do not indicate if the milk is poor or rich, but this can be determined by the lactometer, an instrument for ascertaining in a simple way the specific gravity. The lactometer is shown in Fig. 7.