[ -6 ] 

 IX. Intelligence ami Miscellaneous Articles. 



ON THE INFLUENCE OF PRESSURE ON THE LINES OF THE 

 SPECTRUM*. BY M. L. CAILLETET. 



n^HE experiments which I have the honour of making known to the 

 -*- Academy are founded on the spectroscopic examination of the 

 induction spark produced between two platinum wires sealed to the 

 upper part of a tube of thick glass, in which a gas can be compressed 

 to an exactly determined pressure. 



The pressure is furnished by the apparatus described by me en the 

 occasion of my researches on the compressibility of gases f. The 

 glass tube which contains the platinum wires is joined to a reservoir 

 in shape like a thermometer ; it thus contains a relatively consider- 

 able quantity of gas, which, when driven back by the mercury into 

 which the open part of the reservoir dips, will occupy a height of 

 several centimetres around the platinum wires. The apparatus 

 containing the gas under experiment is enclosed in the laboratory 

 tube of steel, and the upper portion of the glass tube is alone visible. 

 To avoid accidents from its breaking, this tube is coveredby a sheath 

 of very transparent crystal. In case of explosion, as I have often 

 proved, the projected fragments cannot break this envelope, which 

 shelters the observer from all danger. 



When, between the platinum wires, 2 or 3 millims. apart, the 

 spark produced by a RuhmkorfF's coil excited by three Bunsen's 

 elements is caused to pass, the light is very feeble and presents in 

 the spectroscope pretty sharp lines standing out upon the scarcely 

 illuminated background ; if then the pressure be given slowly and 

 gradually, the lines soon become brighter and brighter, then widen, 

 become less sharp, and at last dissolve in the field of the spectrum, 

 which has become brilliant and vividly coloured. At that moment, 

 if the pressure be still augmented, the electric liglit suddenly ceases. 



I made many attempts to remedy this inconvenience ; but my 

 efforts were not successful. I sought especially to avoid the deposit 

 of a very small quantity of alkaline water which constantly condenses 

 on the sides of the glass tube ; for this purpose I lined the interior 

 of the tube with a varnish of gum lac, and inserted a piece of caustic 

 potash. These various expedients did not sensibly retard the moment 

 of the cessation of the passage of the spark. I augmented the in- 

 tensity of the electric current employed ; the spark furnished by a 

 coil of 3 decims. length excited by 8 large Bunsen elements had not 

 sufficient energy to clear the space of less than -^ millim. which 

 separates the platinum wires. 



A stronger electric current, by the sudden heating it produces, 

 inevitably determines the breaking of the glass tube. It is at about 

 40 to 50 atmospheres that the sparks cease to pass; the sides of 

 the tube then become feebly luminous in the dark. 



* I had recently, at EoniP, the opportunity of seeing the Kev. Father 

 Secchi ; and i am anxious to thank him for the encouragement and ex- 

 cellent advice he Idudly gave me ou tlie occasion of the experimenld ivhich 

 constitute the subject of tliis >ote. 



t C'jmptc-s Iicn(-tis, rol, Ixx. p, 1131. 



