Ralph is actually talking philosophy.

Do come to our assistance, for he's out o' sight beyond me already!" "What's the matter ?" inquired Jack, coming up, while he endeavoured to scrub his long hair dry with a towel of cocoa-nut cloth.
I repeated my thoughts to Jack, who, I was happy to find, quite agreed with me.

"Your best plan," he said, "will be to put very few animals at first into your tank, and add more as you find it will bear them.

And look here," he added, pointing to the sides of the tank, which, for the space of two inches above the water-level, were incrusted with salt, "you must carry your philosophy a little farther, Ralph.

That water has evaporated so much that it is too salt for anything to live in.