Chapter-opening picture, page 134. A wide sea-shore scene. On the left, blue-green sea water washes over a flat stretch of wet sand; on the right the sand rises into a dry, pale bank. Along the water's edge the wet sand is crusted with rough white patches. In the middle foreground an elderly, balding man in a white dhoti and shawl, wearing round spectacles, bends right down; a long bamboo staff is in his left hand and he is picking up a little of the white crust with his right hand. Two people crouch beside him on the left, also reaching down towards the white patches. Behind them a large crowd stands watching him — men in white kurtas and white caps, women in white and pale saris with the end drawn over the head, one man in a dark waistcoat leaning in, another with a white towel over his shoulder.
People are collecting natural salt from the seashore. Behind them is the sea; in front of them the wet sand is crusted with white salt, and a man in a dhoti is bending down to pick up a handful of it.
The science of the picture is the science of this whole chapter. Sea water is not pure water — it is a solution in which common salt and other salts are the solutes and water is the solvent. Sunshine and wind evaporate the water, but the salt cannot evaporate. As water leaves, the solution left behind becomes more and more concentrated, until it is saturated. Beyond that point every further litre of water that evaporates leaves solid salt behind, and the salt appears as white crystals on the sand. The people are simply picking up what the sea left.