There is something symbolic here, especially in the context of this chapter. Teresa stands allegorically for material enrichment, while the sea stands for Liberty. The subtle suggestion is that the two cannot be mixed, a suggestion being repeated emphatically across this whole section of the novel. Conrad is arguing that material enrichment (which the workers have) is accompanied by the loss of Liberty; he illustrates it both politically with the anti-capitalist uprising, and personally in the vivid plight of Nostromo. Giorgio's label of "Old Viola" signals the "old" libertarian belief that Wealth and Liberty could co-exist. Note that we are not told who or what thwarts his "desire" to mingle them, encouraging us to read at the level of abstract social forces.