98 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



position of rocks on many definite points, and his researches 

 at once gained recognition. Italian geologists applied them- 

 selves with fresh zeal to the study of their volcanic rocks, 

 working more by the practical methods of Dolomieu. Soon 

 they discovered the weaknesses in Dolomieu's writings, where 

 that keen observer had ventured to speculate on the causes 

 which might determine the particular setting and orientation 

 of mineral material characteristic of the transitional varieties of 

 igneous rocks. 



The learned Lazzaro Spallanzani (1729-99), Professor of 

 Natural History in Pavia, was the first who applied experi- 

 mental methods to the elucidation of volcanic rock-structure. 

 He set up series of experiments in his laboratory in order to 

 find out whether gaseous vapour would escape when lava was 

 melted, and what was the chemical nature of such vapours. 

 The result showed that little gas escaped, but the powdered 

 lava partially sublimated, and was partially converted into a 

 vesicular rock-mass. 



Spallanzani then tested Dolomieu's idea that the crystalline 

 structure of volcanic rocks was produced under the influence of 

 a moderate degree of volcanic heat acting during a long period. 

 Different kinds of lava were exposed to definite tempera- 

 tures for forty-five days, some even for ninety days. The 

 result of Spallanzani's experiment appeared negative, since a 

 moderate heat acting for a long time produced precisely the 

 same effects as a more intense heat acting for a shorter period. 



Spallanzani also investigated whether, in accordance with 

 the hypothesis of Dolomieu, the presence of sulphur would 

 hasten the fluidity of the lava, and whether the melted material 

 in this case would solidify as a crystalline, rough-grained, or 

 vitreous rock. The result was again negative. The powdered 

 specimens of lava mixed with sulphur demanded the same time 

 to become fluid as the specimens with which no sulphur had 

 been mixed, and on solidifying produced the same glassy rock. 

 Spallanzani therefore opposed Dolomieu's theory, that a com- 

 bustible substance was present in flowing lava, pointing out 

 (i) that no flames had ever been seen on the surfaces of lava 

 streams; (2) that all lavas were easily brought back to a fluid 

 condition ; whereas if Dolomieu were right in supposing they 

 became solid after all the combustible material had been con- 

 sumed, then in the absence of the latter it should be much 

 more difficult to melt the lavas. 



