In accordance with the law discovered by Graham—that a colloid solution, such as albumen, cannot pass through a colloidal septum—the blisters cannot be the _result of expansion_. Further: where a colloid septum is wetted on both sides, liquids of the nature of water can diffuse through, and therefore the blisters would be filled with water or solution of crystalloid salt, such as hyposulphite of soda.
Now we must search deeper and wider for the cause. If I am not mistaken, the blisters show themselves in greater force, in this country, from the middle of May to the end of July. During this period the natural waters are of the most favourable temperature for the absorption of air or gases on which the aquatic plants feed; gases coming in contact with or passing through the cells of the living plant are decomposed, and the residual gas discharged into the water. As the water becomes lowered in temperature it is unable to hold these absorbed gases, the minimum temperature being from 32° to 40° Fah.
It is from this cause, in my opinion, that ice floats on the surface. As water becomes lower in temperature minute bubbles of air are seen to grow, as it were, on submerged objects; eventually the buoyancy overcomes the force of adhesion and gravity, whereupon the air-beads rise and become entangled amongst the water crystals—ice—and thus render the ice lighter, bulk for bulk, than the water from which it has been formed.
Some of my readers will, most likely, say—“What has this to do with the blistering of albumen surfaces?” Well, let us see how these ideas, derived from anterior observations, assist us in the matter.
There are many substances beside the growing tissue and cells having the property of condensing gases in their pores, and often entering into combination with such avidity that sufficient heat is liberated to render the substance incandescent. Such is the case when hydrogen gas is impinged on platinum in a spongy form. Fresh-burned charcoal is another substance having the property of condensing many times its own volume of gas, and by mutual attraction to the occluded air the external air sticks as a film on the substance with considerable force—wets the surface, if I may use the simile. Most substances possessing extremely minute pores or interstices have this property, in a more or less degree, for different kinds of gases; therefore we may fairly class the fine texture of photographic paper amongst such bodies.