t + On the Irritability of Sea Sponges. 



produced very able and expert aflronomers, who difcovereoV 

 this period by an aftonifhingly long feries of observations. 

 In this manner every period, however afiumed at pleafure, 

 might be juftified. It would be only neceffary to reckon 

 what length of year it would require •, to afiume a certain 

 acceleration in the revolution of the earth •, to determine 

 thereby at what time the year had the requifite length j 

 and then to be aftonifhed how accurate mankind were ia 

 their aftronomical calculations at that period. 



The reader may determine between both thefe explana- 

 tions. It might dill be fuppofed, that the ancients really 

 had a period of exactly 600 years, neither more nor lefs, and 

 that they deduced it from a falfe determination of the length 

 of the year. But this would not agree well with the reft 

 of their known periods. 



IX. On the Irritability of Sea Sponges. From Neue Nordifche 

 Beytr'age, by Profejfor PALLAS. 



I 



HAVE for a long time remarked in different kinds of 

 fponge ffpongiajt particularly the fpongia villofa> and all 

 tough fea fponges, and even the commoneil fponge em- 

 ployed in bathing-houfes, when not too much ufed, certain 

 remains^of irritability, which, in thefe bodies kept fo long 

 out of their own element, cannot be confidered but as me- 

 chanical. If the above fponges be foaked in cold water 

 until they extend to their full magnitude,, and hot water be 

 poured over a part, or the whole of them, they inftantane- 

 oufty contract themfelves, particularly in the irritated part, 

 with a wonderful velocity and force, as if ;dive. As foon, 

 however, as the irritation of the heat ceafes, thsy again ex- 

 tend to their former fize. This experiment may be repeated 

 with a fponge, until its whole confidence and texture are 

 ieftroyed by the hot water. An accidental obfervation made- 



on 



