MICROSCOPIC DRAWING AND ENGRAVING. 
adjustment is provided, by which the distance between the 
plates of glass may be varied within certain limits, so that by 
turning the screw in one way, the plates may be brought into 
absolute contact, and by turning it the other way, they may be 
separated by any desired interval. Over this cell, is provided a 
small cup, with a hole in its bottom, by means of which the cell 
may be filled with milk. Let us now suppose this cup to be filled 
with the milk to be tested, the screw having been previously 
turned until the plates of glass composing the cell are in contact. 
The milk in that case, will not pass between them, but will 
remain in the cup. Let the observer, applying his eye to the 
small end of the instrument, look through the cell at the flame of 
a candle, placed at about three feet distance from it, and let him at 
the same time slowly turn the screw, so as to let the milk flow 
into the cell; at first the candle will be seen dimly through the 
milk, but when the plates have been separated by the screw to a 
certain distance, the flame will be no longer visible, being inter¬ 
cepted by the multitude of butter globules in the milk. 
Now it will be found, as may be expected from what has been 
explained, that the poorer the milk is, the greater will be the dis¬ 
tance to which the glasses must be separated in order to intercept 
the flame, and the richer it is, on the other hand, the less will be 
the distance which will suffice to produce that effect. 
These instruments are made and sold by the Paris opticians. 
98. It may be objected that the certainty of this instrument 
depends upon the fact that the milk is impoverished either by 
skimming it or by mixing it with water, but that if it be adul¬ 
terated by any substance which will promote its opacity, the 
indications of the instrument must fail. The answer to this 
objection is, that such a mode of adulteration is impracticable; 
the substance used for such a fraudulent purpose must in the first 
place be one, which, when mixed with the milk, will not sensibly 
alter its conspicuous and well-known properties, such as its 
colour, taste, odour, and general consistency. It must, moreover, 
be soluble in the milk, and not merely mixed with it, since if so, 
it would either sink to the bottom, forming a sediment, or rise to 
the top, as oil would in water, and in either case, would be 
immediately detected. It must also be such as will not be 
disengaged by heat, and thereby be betrayed in boiling the milk: 
in fine, it must obviously be a substance cheaper than milk, and 
the process of combination must be so simple as to be inexpensive 
and to admit of a certain secrecy ; now it is quite apparent, that 
there is one substance only which will fulfil all these conditions, 
and tnat substance is water. 
99. The frauds practised by the vendors of milk do not always 
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