1848.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



S39 



The. second series of experiments was tried with an apparatus 

 differing but little from the one just described. The reservoir was 

 a glass globe of from 350 to -tOO cubic centimetres, soldered to a 

 thermometer stem, about 38 centimetres long ; upon this thermo- 

 meter-tube was soldered, at the distance of 1 1 centimetres from the 

 bulb, a piece of tube very regular in its diameter, about 50 

 millimetres long, and of a diameter sufficiently large to present 

 but feeble capillary action. The thermometer-tube was bent at 

 right angles, and drawn out to a point. The first operation was to 

 gauge the apparatus carefully, and to ascertain its co-efficient of 

 dilatation. This was done by filling it with mercury at the 

 temperature 0°, then submitting it to a temperature of 100^ col- 

 lecting the mercury expelled, and weighing this, and the quantity 

 which remained in the bulb. 



The dilatation of the air was then determined very much as 

 before. Eighteen experiments were tried in this way, tlie mean 

 of all of which was 1'36633 ; the maximum, 1-36708; minimum, 

 1-36585 ; the difference, 0-00123, or ^t'tt of the mean. 



The third series of experiments was performed with an apparatus 

 imitated from that described by Rudberg in his second memoir. 

 Upon a shelf within a copper alembic, the cover of which is firmly 

 fixed upon an a|ipropriate support, is placed a glass cylindrical 

 reservoir, 35 millimetres in diameter and 170 millimetres long; 

 to its upper extremity is soldered a thermometer-tube, which passes 

 through a tubulure in -the cover, and bending twice at right 

 angles, is soldered to a larger tube, which dips down into a cistern 

 of mercury, passing air-tight through a tubulure in its cover. On 

 the same shelf is placed a precisely similar reservoir, terminating 

 in a straight thermometer-tube, which passes through another 

 tubulure in the cover, and this apparatus being properly filled 

 with mercury, furnishes a delicate thermometer for noting the 

 temperatures in the alembic. The mercurial cistern is furnished, 

 in its lower part, with a piston, moveable by a screw. Through a 

 second tubulure in the cover of the cistern, passes a straight 

 gauge-tube, open above, and di|)ping into the mercury below, and 

 of the same diameter as the tube which terminates the thermome- 

 ter-stem. The capacity of this apparatus having been gauged, 

 and the co-efficient of dilatation determined by a previous expe- 

 riment, the reservoir is filled with dry air, and the alembic filled 

 with ice, so as to reduce the temperature to 0° ; the piston in the 

 mercurial cistern is then raised or lowered, until the mercury in 

 the tube communicating with the reservoir, stands exactly at a 

 mark previously made upon it, and the difference between this 

 point and the top of the column of mercury in the gauge tube, is 

 measured. The ice is then removed frimi the alembic, and 

 replaced by water, which is boiled, and the temperature of the 

 reservoir being thus brought to 100°, the piston is again adjusted, 

 so as to bring the mercury to the same height as before in the 

 tube communicating with the reservoir, and the differences of its 

 height in this tube and the gauge-tube again read. These two 

 readings of course give the elastic force of the air at these tem- 

 peratures, and from these the co-efficient of dilatation is 

 deduced. The experiments tried with this apparatus, give a 

 mean of 1-36679 — the diffei-ence between the maximum, 1-36747, 

 and the minimum, 1-36612, being toIjt "f the mean. M. Regnault 

 does not believe this method susceptible of the same aecui-acy as 

 the other, on account of the irregular action of capillarity in the 

 tubes, although purposely taken of equal diameters. He also 

 remarks that the results obtained by him are larger than those 

 got by Rudberg from a somewhat similar apparatus, wliich he 

 believes may be attributed to the latter having made his mark 

 npon a capillary tube, and to his neglecting the small (juantity of 

 air contained in the thermometer-tube, which is not heated to 

 100°. As however, unfortunately, M. Rudberg has not stated the 

 dimensions of his apparatus, it cannot be ascertained what influence 

 this had u])on his results. 



For the fourth series of experiments a form of apparatus was devised 

 similar in principle to that just described, but free from its ob- 

 jections. This consisted essentially of a glass globe, of a capacity 

 of from 800 to 2,000 cubic centimetres, to which was added a 

 capillary stem about 20 centimetres long. The globe was placed 

 in an appropriate metallic vessel, so that it could be alternately 

 heated to 100°, and cooled to 0°; the tube passing out of a lateral 

 opening terminated in a small copper pipe which had two other 

 openings — one of these was for the moment closed, the other com- 

 municated with the apparatus for drying the air, by whose means 

 the globe and tube were filled with dry air with the usual precau- 

 tions. Another glass tube of 16 or 17 millimetres internal diameter 

 was cemented at its lower end into an iron cap terminated below 

 by a stop-cock, and carrying a lateral branch bent parallel to the 

 axis of the tube : into this lateral branch was cemented a second 



tube, which was for a certain distance of the same diameter as the 

 first, and terminated above by a capillary tube, a part of that 

 which formed the neck of the globe, which was bent at right angles. 

 This system of tubes being firmly and carefully adjusted in a ver- 

 tical position, the second tube with its attached capillary branch 

 was carefully dried and filled with boiled mercury, and the upper 

 part of the capillary tube, which was of course horizontal, was then 

 fitted into the third opening of the small copper tube, so as to be 

 in immediate communication with the neck of the globe. When 

 firmly fixed, the stop-cock at the bottom of the compound tube 

 was opened, and the mercury flowing slowly out was replaced by 

 air drawn througli the drying apparatus, aiid the apparatus filled 

 with air to a certain mark o, placed upon the vertical tube, where it 

 was of greatest diameter, the glass globe being all the time im- 

 mersed in boiling water : the drying apparatus was then removed, 

 and the branch of the copper tube with which it communicated 

 hermetically sealed, and the height of the barometer noted. The 

 hot water was then discharged from around the globe, and replaced 

 first by cold water and afterwards by pounded ice, the level of the 

 mercury being kept at a, by suffering it to flow oft' when necessary 

 by the stop-cock. When the globe has certainly reached the tem- 

 perature of the ice, the barometer is read, and the difference of 

 the heights of the mercury in the two communicating vertical 

 tubes is measured. We have thus all the data for calculating the 

 co-efficient of dilatation of the air, but another observation may be 

 had by reversing the experiment. To do this, re-connect the dry- 

 ing apparatus with the copper tube, the mercury will fall in the 

 vertical tube in connection with the globe, but must be kept at o 

 by poui-ing mercury into the other vertical tube : when equilibrium 

 has been attained, remove the drying apparatus, and close its 

 branch of the copper tube, then replace the ice by boiling water, 

 and repress the dilatation of the air by pouring more mercury into 

 the vertical tube ; when you are satisfied that the air has taken 

 the temperature of boiling water, read the barometer, and measure 

 the difference of the heights of the mercury in the two vertical 

 tubes. 



The mean of six experiments tried in this way gave 1-3665 for 

 the co-efficient of dilatation : the maximum result being 1-36710 ; 

 the minimum 1-36580 ; difference -j-oVx of the mean. 



By this method the dilatation of the air is determined under 

 very different pressures ; in fact, during the first period of every 

 experiment, the air is under the atmosplieric pressure 0-760 metre 

 when at 100°, and only under the pressure of 0-550 metre when at 

 0°. In the second period, the air at 0°, is under the atmospheric 

 pressure 0-760m., and when heated to 100° under the pressure of 

 about 1-040. It is even easy to arrange the apparatus so that the 

 experiment may be tried under still greater differences of pressure. 

 As the experiments showed no difference in the numbers obtained 

 during these periods (136655 during the first, and 1-36645 during 

 the second period,) we must conclude that within these limits of 

 pressure, the co-efficient of dilatation of air is sensibly constant. 



JFifth series of experiments. — In all the experiments hitherto 

 described, the dilatation of the gas was determined indu-ectly from 

 a direct measurement of the augmentation of its elastic force when 

 brought to a constant volume at a higher temperature, assuming 

 the truth of the law of Jlariotte. In order to get the dilatation 

 directly, the gas enclosed in an eminently elastic envelope should 

 dilate freely without changing its elastic force, and the augmenta- 

 tion of volume must be carefully measured, the gas being all at the 

 same temperature. It is difficult to see how these conditions can 

 be realised in practice, but it may be done approximately by fol- 

 lowing the method adopted by M. Pouillet in his air pyrometer. 

 (Traite de Physique, 4me edit, tome 1, p. 255.) In this way the 

 elastic force of the gas remains sensibly the same, but a very nota- 

 ble portion in the reservoir of dilatation is at a temperature but 

 slightly differing from tliat of the surrounding air. 



The apparatus used by j\I. Regnault was to a great extent similar 

 to that just described, but the iron cap into which the two vertical 

 tubes were cemented was differently adjusted. It had two stop- 

 cocks, by one of which the barometric tube could be made to com- 

 municate at pleasure «ith the exterior, while the other, which was 

 placed under the tube in communication with the globe, was so 

 bored that it might make a communication either between the two 

 tubes or between this second vertical tube and the external air. 

 These two tubes were placed in a glass vessel which could be filled 

 with water so that they could be maintained at any and a uniform 

 temperature. The experiment was conducted as follows : The 

 globe being surrounded witli ice, and the communication with the 

 drying apparatus opened, tlie level of the mercury was brought to 

 the mark a on the vertical tube ; the communication between the 

 two tubes being open, the mercury would of course be at the same 



