SECT. 3] AND EIGHTEENTH CENTURIES 187 



corresponding figures for the adult were: liver 7192, and heart 7836. 

 These figures do not widely diflfer fi:-om those obtained in recent times. 



J. B, Mazin published his Conjecturae physico-medico-hydrostaticae de 

 respiratione foetus in 1737 and his Tractatus medico-mechanica in 1742. 

 In the first of these works Mazin supports what is essentially Mayow's 

 theory of embryonic respiration, without, however, mentioning 

 Mayow more than once. It had not been popular since 1700, though 

 Pitcairn had defended it. Mazin put the liquids of eggs under an 

 air-pump, and observing that air could be extracted from them 

 affirmed that the air was hidden in them and that the embryo could 

 therefore respire. He spoke of "aerial particles" in the amniotic 

 liquid, and discussed the respiration of fishes in connection with this. 

 The specific gravity of the embryo also interested him, and he did 

 a great deal of calculation and experiment on it. His most interesting 

 passage, perhaps, is that in which he mentions the "Eolipile" of 

 the Alexandrians, the primitive form of the steam-engine, and says 

 that just as the heat of the fire makes the water boil, so the heat of 

 the viscera makes the amniotic liquid boil, giving off respirable 

 vapours. The time-relations of this analogy are interesting, for in 

 1705 Thomas Newcomen had succeeded in making a steam-engine 

 which worked with considerable precision, and the question of steam- 

 power was widely discussed. Possibly Mazin was acquainted with 

 the Marquis of Worcester's Century of the Names and Scantlings oj 

 Inventions, which had been published in 1663, and which had con- 

 tained an aeolipile or "water-commanding machine". England was 

 the centre of this movement and other countries employed English- 

 men as engineers; Humphrey Potter, for instance, erected a steam- 

 engine for pumping at a Hungarian mine in 1 720. 



As for the discovery of oxygen, it was near at hand, and Scheele in 

 1 773 and Priestley in 1 774 were soon to supply the knowledge without 

 which Mazin could not proceed further. 



In his second book, Mazin reported many quantitative observations 

 on the specific gravity of the embryo. He found that it diminished 

 as development proceeded, being to the amniotic liquid as 282 to 

 274 in the fourth month and as 504 to 494 in the fifth month. 



Another instance of the way in which experimental physical ques- 

 tions now began to come in is afforded by the work of Joseph Onymos, 

 whose De Matura Foetu of 1 745 spoke of the specific gravity of the 

 embryo at different stages of development. 



