280 CLEAN MILK 



also; or they may be placed loose in cylindrical (better square) copper or tin boxes 

 (with lapped or folded, not soldered seams) a little longer than the pipette ; or 

 they may be simply wrapped individually in filter paper. However prepared, they 

 should be sterilized in the hot air oven for one hour at l6o° C. (or 200° C. for 30 

 minutes), or until a wad of cotton placed beside them as an indicator becomes slightly 

 browned by the heat. 



Flasks (preferably of the Erlenmeyer pattern) of good Bohemian glass with wide necks 

 and flat thin bottoms, 1 liter capacity. Plug with cotton and sterilize in hot air. 



Bottles — 8 oz. Blake's, square, wide mouthed, marked (by a file, or diamond, but best 

 with hydrofluoric acid) at 99 c.c. capacity for dilution water. Plug with cotton and 

 sterilize in hot air. _ 



Petri dishes — 4" (10 cm.) glass bottoms,- with porous covers to match. Wrap in soft 

 paper or place in sets of 10 in a covered cardboard box for sterilization by dry heat. 



Incubator. If gas is available, a Koch safety burner and Lautenschlager ether-mercury 

 gas regulator may be used. Fair results have been obtained where gas was not 

 available by using an ordinary chicken incubator carefully regulated and watched 

 daily. 



Counting apparatus. That of the International Instrument Co. (Boston Board of Health 

 pattern) or Stewart's counter, are good. A home-made one quite efficient for the 

 finest work may be made from a child's school slate, on which is scratched a four- 

 inch circle, divided by four cross lines into eighths ; a box about 4J to 5 inches square, 

 about 4 inches high, without a bottom, to rest on the slate over the square, and 

 having one side (to face the window) glass, the top perforated by a hole 3$" in dia- 

 meter, on which a 4" reading glass rests ; the height of the box to correspond 

 with the practical focus of the lens, which may be determined readily before- 

 hand (/'. e„ the distance which the lens should be from the object examined 

 in order to give a clear magnified image to the observer with his eye at a convenient 

 distance from the lens). To count a plate, place the plate upon the circle marked 

 on the slate, centering it carefully ; remove the cover ; place the box over it, with, 

 the glass front towards the window and the reading lens over the circular opening 

 in the top : and count all the colonies visible, using the divisions of the circle merely 

 as an aid to prevent recounting the same colony. It is well to number the divisions 

 and always to begin counting at division No. 1, in order that the same division 

 should not be counted twice. Of course, once the counting has begun, the plate 

 must not be permitted to shift on the lines even to a very slight extent. 

 I Microscope. A microscope is not essential to bacterial work on milk, so long as species 

 differentiation is not attempted, and much valuable information bearing on clean 

 milk can be demonstrated by a beginner to himself through methods hereinbefore 

 described without ever seeing an isolated bacterial cell — in other words, by dealing 

 wholly with mass growths of bacteria, cultures, colonies, etc. If a microscope is de- 

 sired, however, a. good instrument should be bought from standard makers, Ameri- 

 can or foreign— a monocular tube, with fine adjustment, substage condenser, iris 

 diaphragm and oil-immersion lens covering the essentials. A combination of lenses 

 giving great satisfaction in practice consists of: objectives,^ oil immersion; J 

 " dry," and J "dry" ; and oculars Nos. 1 and 4 or equivalents. A triple nosepiece is 

 not necessary, although a great convenience ; a mechanical stage is needed only 

 for refined work. A satisfactory instrument for all ordinary work may be had at 

 from $70 to JS100. Books describing the care and use of the microscope may 

 be had, notably by Gage and by Wright (Gage—" The Microscope," Comstock 

 Publishing Co., Ithaca N. Y. ; Wright — " Principles of Microscopy," MacMillan Co., 

 New York). 



