[21] la substitución del vapor por la electricidad, the substitution of electricity for steam. Note transposition of terms.
[22] Pues, porque..., Well, because....
[23] a manera de..., by way of (reply).
[24] casi libre de todo rastro de, almost free from any trace of.
[25] guiñada de inteligencia, a knowing wink.
[26] tomando la broma en serio, taking the joke seriously.
[27] además de ser un abono excelente, es un elemento indispensable..., besides being an excellent fertilizer, is an indispensable element (constituent) .... VARIANT: Además de un abono excelente, es un elemento....
[28] rascándose la cabeza, scratching his head. Note the article instead of the possessive.
[29] eso me gusta menos, I don’t like that so well.
[30] de una substancia tan preciosa, of so precious (valuable) a substance.