372 



position, which rapidity of fusion is diminished by the more rapid 

 removal of the heated gases from the surface of the composition. 

 Hence the rate of burning of combustibles of this class depends upon 

 the pressure of the medium in which they are consumed. In the 

 case of time-fuses, the increments in the time of burning are propor- 

 tional to the decrements in the pressure of the surrounding medium. 



3. The luminosity of ordinary flames depends upon the pressure 

 of the supporting medium ; and, between certain limits, the decrease 

 in illuminating power is directly proportional to the decrease in 

 atmospheric pressure. 



4. The variation in the illuminating power of flame by alterations 

 in the pressure of the supporting medium depends chiefly, if not en- 

 tirely, upon the ready access of atmospheric oxygen to, or its com- 

 parative exclusion from, the interior of the flame. 



5. Down to a certain minimum limit, the more rarefied the atmo- 

 sphere in which flame burns, the more complete is its combustion. 



XVI. " On the Anatomy and Physiology of the Spongiadse." 

 Part II. By J. S. BOWERBANK, LL.D., F.R.S., F.L.S., 

 &c. Received June 17, 1861. 



(Abstract.) 



This paper is a continuation of the first division of the subject 

 published in the Phil. Trans, for 1858. 



In the second part of this division the author treats of the kera- 

 tode or horny substance of the skeleton, as regards both its physical 

 and chemical characters, with a view of establishing the animal na- 

 ture of that substance. 



In the third part the membranous tissues are described under 

 two heads : 



1st. Simple membranous tissues analogous to those of the base- 

 ment membranes of the higher classes of animals ; and 



2nd. Compound membranous tissues. These structures consist 

 of simple membranous tissue combined with primitive fibrous tissue. 

 Their most simple forms exist in the membranes lining the intersti- 

 tial cavities of the sponge and in the dermal membrane. 



