small quantity, that color is transmitted to the eye so faintly as to be inefficient j 

 in producing perception. 



The water of the sea exhibits another remarkable example of this effect, i 

 If we look into the sea where the water has considerable depth, we find that ] 

 its color is a peculiar tint of green ; but if we take up a glass of the water i 

 which thus appears green, we shall find it perfectly limpid and cold-less. The ) 

 reason is, that the quantity of the color is too small to be perceivable ; while the 

 great mass of water, viewed when we look into the deep sea, throws up the 

 color in such abundance as to produce a strong and decided perception of it. 



The atmosphere is in the same circumstances ; the color, from even a con- 

 siderable portion of it, is too faint to be perceptible. Hence the air which 

 fills an apartment, or which immediately surrounds us when abroad appears 

 colorless and transparent. But when we behold the immense mass of atmo- 

 sphere through which we view the firmament, the color is reflected with suffi- 

 cient force to produce distinct perception. But it is not necessary for this that 

 so great an extent of air should be exhibited to us as that which forms the 

 whole depth or thickness of the atmosphere. Distant mountains appear blue, 

 not because that is their color, but because it is the color of the medium through 

 which they are seen. 



Although the preceding observations belong more properly to optics than to 

 mir present subject, yet still, since the exhibition of color is one of the mani- 

 festations of the presence of body, they may not be considered as foreign to 

 an investigation of the mechanical properties of atmospheric air. The mind un- 

 accustomed to physical inquiries finds it difficult to admit that a thing so light, 

 attenuated, impalpable, and apparently spiritual as air, should be composed of 

 parts whose leading properties are identical with those of the most solid and ada- 

 mantine masses. The knowledge that we see the air must, at least, prepare 

 the mind for the admission of the truth of this proposition that " air is a body." 



WEIGHT OF AIR. 



Among the properties which are observed to appertain to natter, and which 

 as far as we know are inseparable from it, in whatever form, and under what- 

 ever circumstances it exists, weight and inertia hold a conspicuous p-lace. To 

 be convinced, therefore, that air is material, we ought to ascertain whether it 

 possesses those properties. We shall have frequent and numerous proofs of 

 this ; but it will at present be convenient to demonstrate it in such a manner 

 that we shall be warranted in assuming it in some of the explanations which 

 we shall have to offer. 



The most direct proof that air has weight, is the fact that if a quantity of it 

 be suspended from one arm of a balance, it will require a definite weight to 

 counterpoise it in the opposite scale. By the aid of certain pneumatical en- 

 gines, the nature of which will be explained hereafter, but the operation and 

 effects of which will for the present be assumed, this may be experimentally 

 established. 



Let a vessel containing about two quarts, be formed of thin copper, with a 

 narrow neck, in which is placed a stop-cock, by turning which the vessel may 

 be opened or closed at pleasure. Let two instruments be provided called syr- 

 inges ; one, the exhausting syringe, and the other the condensing syringe. 

 Let the exhausting syringe be screwed upon the neck of the vessel and let the 

 stop-cock be opened so that the interior of the vessel shall have free communica- 

 tion with the bottom of the syringe ; if the syringe be now worked, a large portion 

 of the air contained in the vessel may be withdrawn from it. When this has 

 been done, let the stop-cock be closed to prevent the re-admission of air, and lot 



