234 Geological Society : — ^Prof. A. Daubree on 



in series was 1 : 2, 1 : 3, . . . of that for a single jar. The illu- 

 mination, however, was always greater \sdth the larger charges, i. e 

 with the greater quantity of electricity discharged. 



The experiments above described were made first with tubes in 

 which the pressure was moderately high. They were afterwards 

 repeated with lower pressures, and results of the same character as 

 before were obtained. But, owdng to the smaller amount of the 

 critical charges, to the greater extension of the negative glow, and 

 to the consequently increased delicacy of the phenomena, the same 

 numerical precision was not attained. But there seems no reason 

 to doubt that the discrepancies might be indefinitely diminished by 

 instrumental refinements. 



The duration of the stratified discharges observed throughout 

 these experiments was exceediugly short, indistinguishable, in fact, 

 from that of the true jar-discharge. When viewed in a revolving 

 mirror, either with or without a slit, they showed no sign whatever of 

 prolonged duration ; and we may thence conclude that, so far as our 

 present instrumental arrangements extend, there is no inferior limit 

 to the duration of discharge necessary for the production of striae. 



In connexion with this part of the subject another form of ex- 

 periment was arranged. Beside the jars hitherto described another 

 was used, having its inner surface connected with one terminal of 

 the tube, and its outer with the other. When this disposition was 

 made, the additional jar acted as a buffer, and produced a stratified 

 discharge under circumstances which would without it have pro- 

 duced a true jar-discharge. 



A comparison of the results here obtained with those detailed in 

 Part II. of these researches shows that the phenomena produced 

 by suitable disposition of the Ley den battery coincide with those 

 produced by the induction-coil. With the coil it was found that 

 (1) for a given electromotive force the column of striae was shorter 

 the larger the battery-surface or strength of current used ; (2) that 

 the proper motion, when directed, as usual, towards the positive 

 terminal, was more rapid the greater the electromotive force 

 employed. With the Ley den battery (1) it was found that, in 

 order to maintain the same length of column with an increased 

 surface, the charge must be increased in a larger proportion than 

 the surface ; and (2) it was noticed that the striae, which when the 

 tension was low were distinct and well separated, became more 

 blurred as the tension rose, until they sometiuies w ere blended into 

 an apparently unbroken column of light. The presence, however, 

 of the negative glow still showed that the true jar-discharge had not 

 yet been reached. 



GEOLOGICAL SOCIETY. 



June 20th, 1877.— Prof. P. Martin Duncan, M.B., F.R.S., President, 



in the Chair. 

 [Continued from p. 153.] 

 5. " On roints of Similarity between Zeolitic and Siliceous Incrus- 

 tations of recent formation by Thermal Springs and those observed 



