In this passage from Chapter 9, Margaret makes an allusion to the Swiss Symbolist painter Böcklin at a lunch party. She does so as part of a complaint about the views of the English on German art. She also uses an idiom which expresses her displeasure with English closed-mindedness:
My blood boils—well, I’m half German, so put it down to patriotism—when I listen to the tasteful contempt of the average islander for things Teutonic, whether they’re Böcklin or my veterinary surgeon. ‘Oh, Böcklin,’ they say; ‘he strains after beauty, he peoples Nature with gods too consciously.’ Of course Böcklin strains, because he wants something—beauty and all the other intangible gifts that are floating about the world. So his landscapes don’t come off, and Leader’s do.
The word “Teutonic” in this context refers to Germany, and was in common usage in the 20th century. Margaret here sarcastically says that the English have a “tasteful contempt” for Germans, whether the Germans in question are famous artists or “veterinary surgeons.” In this case she feels angry that English people dismiss German art as “strain[ing] towards beauty,” and that they say that it’s too “conscious” of its contents. Margaret believes that Böcklin's landscapes do not succeed for the English because he is trying to depict something “intangible.” She implies that this "intangible" is too subtle for the English to grasp. She bolsters this claim using the example of the artist Benjamin Williams Leader, who was very popular in England at the time. She says Leader is more suited to English audiences because his paintings are realistic and don’t have to “strain" toward anything hard.
Margaret expresses her self-righteous anger in this moment by saying her “blood boils.” This phrase conveys an intense internal rage. It also refers to Margaret’s national identity. She is “half-German” and partially attributes her loyalty to German art to patriotism. As she has German blood, it “boils” when she hears about ignorant English contempt. The passage also highlights the tension between English and German culture during the period, at a historical high because of all the developments in science, philosophy, and politics happening in both places.
It’s worth noting here that Böcklin is actually not a German painter. He’s Swiss. It seems that either Forster didn’t know that, or he intended Margaret to be—ironically—expressing some English ignorance herself.