CHEMISTRY. 



157 



Chemic.il 

 Apparatus. 



Mercurial 

 troughi. 



PtiTB 

 OXLII. 

 F'f. 5. 



Method of 

 measuring 

 the buJk of 



be essentially referred either to one or other of the two 

 w hich we have figured. 



5. Many gases are absorbed with such avidity by 

 water, that it is necessary to prepare them over mer- 

 cury. Hence a mercurial trough is also requisite for all 

 those who wish to investigate the properties of gaseous 

 bodies with any thing like success. Mercurial troughs 

 are very various in their shapes. One of the most usual 

 and best shapes is represented in Fig. 5. It may be made 

 of mahogany, or it may be cut out in marble ; or what 

 is now most common, it may be of iron, and covered 

 externally with wood, a is the cistern filled with mer- 

 cury, in which the glass jars are to be filled, and then 

 slid upon the shelf visible in the Figure. The glass jars 

 used with this trough must be smaller than those used 

 for the water trough, and they ought to be stout. As 

 they are liable to be overturned by the weight of the 

 mercury, care must be taken to fix them. 



Mercurial gas-holders are used, made on a similar plan 

 with the second kind of gas-holders described above. 

 The outer case is cast iron. There is an inner cylinder 

 of cast-iron, at a little distance within the interval 

 between them, filled with mercury; v, a glass jar, is 

 made to move freely between both, and to receive- the 

 gas. The reader will easily conceive a gas-holder of this 

 kind without a figure. 



6. It is necessary to be in possession of a method of 

 measuring the bulk of our gases with accuracy. For 

 this purpose, the jars into which they are received should 

 be accurately graduated. The best graduation is into 

 cubic inches, and thrir decimal parts. The experimen- 

 ter should perform this graduation himself, otherwise he 

 can very seldom depend upon its accuracy. He should 

 first provide himself with some glass tubes, shut at one 

 end and open at the other, and of as equal a bore as pos- 

 sible. Some of them should be |th inch in width, others 

 jth, and some may be as wide as f ths, or even ^ths of an 

 inch. A long ribbon of white paper, about fth inch 

 wide, should be pasted along the whole length of the 

 tube by means of gum water. That it may adhere to 

 the glass in every part, a quantity of twine may be wrapt 

 round it while wet, and the tube may be set aside till 

 next day, when the paper will be found adhering to it 

 closely in every part. The tube is now set in an exact 

 perpendicular direction with its open mouth uppermost. 

 We (hall suppose it our intention to graduate this tube 

 into hundred part* of a cubic inch. The be*t method 

 of doing this is the following : 



Provide a quantity of mercury, and ascertain its spe- 

 cific gravity with accuracy. This being known, it is 

 easy to ascertain the weight of T-^jth cubic inch of mer- 

 cury. For example, the mercury which we are in the 

 habit of using is of such specific gravity, that -^^ of a 

 cubic inch weighs 34.46 grains. Weigh out this quan- 

 tity of mercury, and introduce it into a tube terminating 

 in a point, and of a bore as narrow as possible, but still 

 allowing the mercury to run out freely. The mercury 

 should occupy about 3 inches in length of it from the 

 pointed end. Mark carefully with a file the part of the 

 tube to wliich the mercury rises. This tube, thus pre- 

 pared with a pair of good scales, will enable you to gra- 

 duate ygur tube with considerable rapidity. Pour the 

 first T^JT cubic inch of mercury into your tube, observe 

 its surface, and draw a fine line with a pencil upon the 

 ribbon of paper over against this surface. Draw up into 

 your pointed tube, by means of the mouth, a quantity of 



mercury, till it just rises to the mark. Put this quantity Chemical 

 into your scales, and, if you manage matters with tole- Appai 

 rable dexterity, you will find that it will weigh exactly *"T"~ 

 34 46 grains. Pour this into your tube, and draw a pen- 

 cil line over against its surface. Proceed in this way till 

 you have filled your tube to the top. By this method, 

 you have got your tube graduated upon the ribbon of 

 paper. To graduate it upon the tube itself, which is re- 

 quisite, procure some small triangular files, and moisten- 

 ing one of the angles, draw it along the pencil marks upon 

 the paper. You soon cut through the paper, and make an 

 impression upon the glass tube ; and, if you manage mat- 

 ters with address, you do not injure the adhesion of the 

 re-t of the paper to the tube. At every 5th degree, 

 make the line upon the tube somewhat longer than the 

 preceding 4 lines ; and make the line still longer at every 

 ]0th degree. Having thus marked the degrees upon the 

 whole length of the tube, dip it jnto water, and wash 

 off the paper. If you have used the file properly, you 

 will now find the lines engraven upon the tube very neat- 

 ly and distinctly. You now with a diamond write the 

 numbars 10, 20, 30, &c. opposite to the respective lines 

 upon the tube. 



Large jars are to be graduated in the same manner, 

 only you divide them into cubic inches instead of ra^-th 

 of cubic inches. It is necessary also to use water instead 

 of mercury. As a portion of the water adheres to the 

 vessel in which you weigh it, you must make an allow- 

 ance for this quantity, otherwise your marks will repre- 

 sent something less than cubic inches. This error, 

 though small, becomes extremely sensible after being re- 

 peated one, two, or three hundred times. 



7. Suppose it required to take a small quantity of gas 

 out of a jar standing over water, and introduce it into a 

 vessel standing on the mercurial trough, without taking 

 any of the water along with it, or even wetting the in- 

 side of the vessel. This may be done by a very simple PLATE 

 instrument, first contrived by Mr Cavendish, and repre- CXLH. 

 sented in Fig. 6. It consists of a glass tube, graduated ^'S- 6 - 

 into T i 5 ths of a cubic inch, open at both ends: One 



end, however, drawn out to a capillary tube. This tube 

 being first tilled with mercury, is to be introduced into 

 the jar standing on the water trough, with the finger 

 shutting up closely the end A, which must be under- 

 most. When fairly within the jar, you withdraw the 

 finger a little from one side of A, and let the mercury 

 run out, till its upper extremity comes opposite to the 

 degree, marking the quantity of gas which you want to 

 take out. The finger immediately resumes its place. 

 By this contrivance, the gas is made to enter at the ca- 

 pillary end B, and to fill what part of the tube you have 

 a mind. You then withdraw the tube from the water 

 trough, holding your finger in its position all the while, 

 and wipe the outside of the tube dry with a cloth. You 

 then carry it to the mercurial trough, place the point B 

 under the jar or tube into which you wish to introduce 

 the gas, and bending the tube su, that the whole of A is 

 under the surface of the mercury, and the end A some- 

 what lower than the end B ; remove your finger from A, 

 the mercury runs in, and the gas makes its escape from 

 the capillary end B, and enters into the vessel prepared 

 for its reception. 



8. The next object after being able to measure the Method O f 

 bulk of gases, is to ascertain their specific gravity. To lllc " surul 

 do this, it is necessary to be provided with a good air ' JjJ^'of 

 pump, or at least with an exhausting syringe. Very ex- 



