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Updated: June 1, 2025
If the acid is exactly neutralised by, that is combined with, the alkaline base to form fully neutralised salts, the litmus paper takes a purple tint. Coloured reagents such as litmus are termed indicators. A substance called phenolphthalein, a coal-tar product, is a very delicate indicator; it is more sensitive to acids than litmus is.
In hair the overlapping scales are attached to the under layer up to the edges of those scales, and at this extremity can only be detached by the use of certain reagents. But this is not so with wool, for here the ends of the scales are, for nearly two-thirds of their length, free, and are, moreover, partially turned outwards.
The third year laboratory is designed for making commercial analyses. These latter are made by either dry or wet way. The first method employs water chiefly as a vehicle, and alkaline solutions as reagents. The second employs reagents in a dry state, and the action of which requires lamp and furnace heat.
Dry cells are more cleanly and portable than wet, they require little or no attention, and are well suited for household or medical purposes. The zinc plate forms the vessel containing the carbon plate and chemical reagents. The black paste is surrounded by a white paste Z, consisting mainly of lime and sal-ammoniac.
She carried the brimming cup of her inestimable virtues with a cautious, steady hand, and an eye always on them, to see that they did not spill. Then she was an admirable judge of character. Her mind was a perfect laboratory of tests and reagents; every syllable you put into breath went into her intellectual eudiometer, and all your thoughts were recorded on litmus-paper.
Surely there must be some scars left after such grafting. I picked up the various sets of prints. It was true that the third set was not very clear, but there certainly were no scars there. "Though there is no natural changeability of finger-prints," pursued Kennedy, "such changes can be induced, as Dr. Paul Prager of Vienna has shown, by acids and other reagents, by grafting and by injuries.
"I purpose to seek ay, and to find the proper reagents, which, acting upon the living organism, will bring about molecular changes analogous to those you have just witnessed. But these reagents, which I shall find, and for that matter, upon which I already have my hands, will not turn the living body to blue or red or black, but they will turn it to transparency. All light will pass through it.
One fact, at any rate, is out of reach of such refinements, and this is, that all the forms of protoplasm which have yet been examined contain the four elements, carbon, hydrogen, oxygen, and nitrogen, in very complex union, and that they behave similarly towards several reagents.
The chemist has his acids, and reagents, and blowpipes, etc.; they constitute his instruments, and by using them, under certain constant rules, he keeps to a consistent method. So with the physiologist; he has his microscope, his staining fluids, his means of stimulating the tissues of the body, etc.
If a difference of action in certain reagents, in ammonia, for example, were sufficient to determine the limits of the animal and vegetable kingdoms, might we not argue that there must be a very great and natural difference between moulds and bacteria, inasmuch as the presence of a small quantity of acid in the nutritive medium facilitates the growth and propagation of the former, whilst it is able to prevent the life of bacteria and vibrios?
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