582 PROCEEDINGS OF THE ACADEMY OF [NoV.^ 



corded. Some are entirely new to the United States, while others 

 are quite rare. 



Among these may be mentioned Actmocyclus barkleyi aggregatu» 

 Rattr., Navicula placenta Ehr., Polymyxus coronalis L. W. Bail, 

 Amphiprora conspicua Grev., Nitzschia clausii Hantzsch. 



Brackish and marine forms occur in the blue-clay deposits in 

 various parts of the city. 



Other rare species were described. 



The entire flora of the city and immediate vicinity numbers 

 probably three hundred distinct species. Illustrations of many 

 diatoms were shown, including some of those found in the city water 

 supply. 



The Critical Point in Liquids. — Mr. Hugo Bilgram remarked that 

 of the various inclusions found in quartz those of liquid dioxide of 

 carbon are among the most interesting. Within more or less irregular 

 microscopic cavities are inclosed three distinctly visible fluids: 

 water, liquid dioxide, and within this a bubble of dioxide vapor. 

 When the temperature is raised, the liquid dioxide is turned into 

 vapor and the bubble disappears. Upon cooling the object, the 

 vapor bubble suddenly reappears. In many cases not a single, but 

 a number of bubbles make their appearance, making it look as if 

 the inclusion were boiling. How is this phenomenon to be explained? 



It is well known that water boils when heated to 100 degrees 

 Celsius; but if exposed to a reduced pressure, its boiling point is 

 lower, and if exposed to a pressure higher than that of the atmos- 

 phere, the boiling point is higher. If the relation of the vapor 

 pressure to the temperature is represented by a curve, it can be seen 

 that at a point somewhat over 300 degrees the pressure curve extends 

 to infinity. This shows that above this temperature water cannot 

 exist in liquid form, no matter how high the pressure, and this degree 

 of heat is termed the "critical point." 



All volatile liquids show the same peculiarity, but the critical 

 point is different for different liquids. That of dioxide of carbon 

 is in the neighborhood of 28 degrees Celsius, and the phenomena 

 above described present the change of dioxide of carbon from the 

 liquid to the gaseous state and vice versa at a temperature which is 

 near the critical point. From the peculiar features of the phenomena 

 very interesting inferences can be derived. 



There is a sharp distinction between fluids and gases, as we know 

 them, the principal differences being in relation to elasticity, cohe- 

 sion, and expansion by heat. While gases are highly compressible 

 and have a high coefficient of expansion by heat, compressibility 

 of liquids is very slight and expansion by heat not very great. While 

 gases readily expand if the space of their confinement is increased, 

 liquids maintain their volume. In gases there is a total absence of 

 cohesion, while in the case of liquids the molecules cohere. When 

 water or any other liquid is converted into steam, the change from 

 the liquid to the vapor state is abrupt. But the phenomena exhibited 



