Images de page
PDF
ePub

and its quantity ascertained. Dissolve the anthrazothionate of manganese in water, and precipitate the manganese by means of potash. From the oxides thus obtained, we may determine the quantity of metal by Berzelius's method; but this method of separating the two metals is not absolutely correct, because anthrazothionate of manganese is slightly soluble in alcohol.

ARTICLE VIII.

A remarkable Combination of Carbonate of Lime and Hydrate of

Lime, observed by Theodor von Grotthuss.* When a strong current of carbonic acid gas is suddenly passed through lime water, there is formed not a pure carbonate of lime, but a mixture of carbonate and hydrate. This fact deserves attention, because in many cases it is easy to mistake one of these compounds for the other. It is very bulky, and falls slowly to the bottom in flocks ; but it has only an ephemeral existence; for as soon as these flocks approach near each other, they lose at once their voluminous appearance, and do not assume it again when agitated. They have now a granular, powdery consistence, and a much greater specific gravity, in consequence of which they sink rapidly to the bottom, and do not appear in flocks. The substance in this last state is pure carbonate of lime. If concentrated acetic acid be poured upon the bulky precipitate when first formed, not the smallest evolution of air bubbles is perceptible; because the carbonic acid as it is set at liberty, finds, in consequence of the great bulk of the hydrate and of the water which it contains, so many points of contact with it, that it cannot assume the gaseous form.

If a concentrated solution of an ammoniacal salt, for example, sal ammoniac, be poured into the bulky hydrate, which renders the water as white as milk, the liquid becomes immediately almost colourless, because the hydrous carbonate passes at once into the state of pure carbonate, which last has a very small volume when compared with the former. Were the bulky precipitate merely a hydrous carbonate, we might suppose that the salt deprives it of its water ; but a concentrated solution of common salt does not, by any means, produce the same appearance. The bulky precipitate, therefore, must be a compound of carbonate and hydrate of lime. The hydrate unites with the acid of the ammoniacal salt and sets the ammonia free, while the pure carbonate only remains behind. The action of the water and attraction of cohesion of the carbonate of lime, appear gradually to destroy the compound. It has, therefore, as has been already observed,

• Translated from Schweigger's Journal, xx, 275,

only an ephemeral existence. It is very easy to form it, merely by blowing out air from the lungs through a glass tube in lime water. Carbonate of magnesia seems to form the same kind of combination ; for, according to Bucholz, it is capable of existing in the state of three different compounds.

ARTICLE IX.

Description of an improved Microscope. (See Pl. LXXXVIII.)

Fig. 1. The instrument mounted for viewing an opaque object. A B is the body of the microscope ; it consists of five lenses, and differs from that commonly used for microscopes. The instrument-maker will comprehend how to make it when he is told that the lenses 1 and 2 at the bottom are similar to those used for the eye-pieces of refracting telescopes; the lenses 3, 4, and 5, are the field-glass and double eye-glass used in compound microscopes. To increase the magnifying power, there are three astronomical eye-pieces of different powers, which are made to fix on at A. This body is eight inches long, the bottom is tapered a little, as may be seen at B, for the purpose of allowing the rays to pass more freely from the mirror $ to the object to be examined. At the top, A, a shoulder is made which is screwed for a purpose hereafter to be mentioned. If it is asked why I have rejected the old body for viewing opaque objects, and what is the advantage of this new one, I answer as follows. Every one accustomed to common compound microscopes may have observed a kind of milkiness in the field, so that the object appears as if seen through a kind of mist, or as if the glasses were dimmed by moisture. The greater the

apertures of the little glasses are at the bottom, the more perceptible this becomes; but it can never be removed altogether, as I know from experience, by any reduction of the apertures : when an opaque object is viewed, the defect is still more striking; besides opaque objects require a great deal more light to be seen

properly than transparent ones, and this kind of microscope gives very little, especially if the apertures are reduced to a proper standard. Our microscope labours under none of these defects; the image is quite clear, as if produced by a single lens, and there is abundance of light. The body of the instrument is made to twist into the socket of the arm CC, which is made to slide backwards and forwards in the case represented at 0. The stage, D D, travels up and down with a rack and pinion, as seen in the Plate; it is five inches in length, reckoning from the brass bar, 000; the hole E is 24 inches diameter; that at F one inch, and has a small notch in it. There are two holes

[merged small][merged small][merged small][graphic][ocr errors][ocr errors][subsumed][ocr errors][merged small][ocr errors][ocr errors][subsumed]

only an ephemeral existence. It is very easy to form it, merely by blowing out air from the lungs through a glass tube in lime water. Carbonate of magnesia seems to form the same kind of combination ; for, according to Bucholz, it is capable of existing in the state of three different compounds.

ARTICLE IX.

Description of an improved Microscope. (See Pl. LXXXVIII.)

Fig. 1. The instrument mounted for viewing an opaque object. A B is the body of the microscope; it consists of five lenses, and differs from that commonly used for microscopes. The instrument-maker will comprehend how to make it when he is told that the lenses 1 and 2 at the bottom are similar to those used for the eye-pieces of refracting telescopes; the lenses 3, 4, and 5, are the field-glass and double eye-glass used in compound microscopes. To increase the magnifying power, there are three astronomical eye-pieces of different powers, which are made to fix on at A. This body is eight inches long, the bottom is tapered a little, as may be seen at B, for the purpose of allowing the rays to pass more freely from the mirror $ to the object to be examined. At the top, A, a shoulder is made which is screwed for a purpose hereafter to be mentioned. If it is asked why I have rejected the old body for viewing opaque objects, and what is the advantage of this new, one, I answer as follows. Every one accustomed to common compound microscopes may have observed a kind of milkiness in the field, so that the object appears as if seen through a kind of mist, or as if the glasses were dimmed by moisture. The greater the apertures of the little glasses are at the bottom, the more perceptible this becomes ; but it can never be removed altogether, as I know from experience, by any reduction of the apertures : when an opaque object is viewed, the defect is still more striking; besides opaque objects require a great deal more light to be seen properly than transparent ones, and this kind of microscope gives very little, especially if the apertures are reduced to a proper standard. Our microscope labours under none of these defects; the image is quite clear, as if produced by a single lens, and there is abundance of light. The body of the instrument is made to twist into the socket of the arm CC, which is made to slide backwards and forwards in the case represented at 0. The stagę, D D, travels up and down with a rack and pinion, as seen in the Plate; it is five inches in length, reckoning from the brass bar, 0 0 0; the hole E is 27 inches diameter ; that at F one inch, and has a small notch in it. There are two holes

[graphic][merged small][subsumed][ocr errors][subsumed][merged small][subsumed][ocr errors][ocr errors][subsumed][ocr errors][subsumed][merged small][ocr errors][ocr errors][subsumed][ocr errors][ocr errors][subsumed][subsumed][ocr errors]
« PrécédentContinuer »