On July 21, 1915, there came a complete lull. The next day the Austrians opened their attack with a concentrated bombardment. During the period of Italian advance the railways had been piling up the Austrian shells and German gunners had been sent by the Crown Prince of Bavaria to help serve the heavy howitzers rushed to the Carso from the Julian Alps and the Tyrol and Trentino salients. With the design to cut the Italian line of communication, the main Austrian infantry attack was delivered toward Gradisca where the Italians had constructed their principal bridges across the Isonzo. The infantry massed behind the neighboring hills and under cover of a tremendous artillery bombardment advanced in close formation. The first line of Italian troops seemed about to be swept away when the gunners on the heights across the river got the range and poured into the advancing Austrians a massed fire from all their 500 pieces. General Boroevics's advance was pounded to pieces; the Italians brought up reenforcements and charged and captured the lines from which the Austrians had delivered their assault, taking 2,000 prisoners. [Illustration: A cloud of poisonous gas released by Italian troops from tanks concealed in the thicket.] On July 23, 1915, the archduke ordered another attack upon the Italian positions near the sea on the edge of the Carso tableland. This was really an effort to recapture Monfalcone; but it failed, although the Italians did not dare risk pursuit over the rough ground. Later two Austrian divisions, advancing from San Michele and San Martino against Sagrado were repulsed with heavy losses. By July 25, 1915, the Italians were able to attack and capture some of the intrenchments on the slopes of San Martino and to storm Sei Busi. This hill of Sei Busi witnessed some of the most sanguinary fighting of the whole series of engagements. On a single day it was won, lost and won again by the Italians, both sides bringing up strong reenforcements and concentrating against the summit all the artillery within range. Over the crest of San Michele which dominated a large part of the tableland the battle surged for many days.